AP Bio Lesson 0 Scientific Method and Experimental Design

AP Biology Lesson 0

• Scientific Method and Experimental Design

LO: Review the basic concepts of the scientific method and experimental design:

• Identify or pose a testable question based on an observation, data, or a model.

• State the null and alternative hypotheses, or predict the results of an experiment.

• Identify experimental procedures that are aligned to the question, including:

  • Identifying dependent and independent variables.

  • Identifying appropriate controls.

  • Justifying appropriate controls.

• Support a claim with evidence from biological

• principles, concepts, processes, and/or data.

• Provide reasoning to justify a claim by connecting evidence to biological theories.

• Explain the relationship between experimental results and larger biological concepts, processes, or theories.

• Construct a graph, plot, or chart.

• Describe data from a table or graph, including: Identifying specific data points; Describing trends and/or patterns in the data.

  • c. Describing relationships between variables.

Four Big Ideas of AP Biology:

• Big Idea 1: Evolution- the process of evolution drives the diversity and unity of life

• Big Idea 2: Energetics- Biological systems use energy and molecular building blocks to grow, reproduce, and maintain dynamic homeostasis.

• Big Idea 3: Information storage and transmission- living systems store, retrieve, transmit, and respond to information essential to life processes.

• Big Idea 4: Systems interactions- Biological systems interact, and these systems and their interactions exhibit complex properties.

The Four Big Ideas are Interleaved into 8 Units and are Separated into:

• Learning objectives (LO): define what a student needs to be able to do with the content knowledge in order to progress through the course

• Essential knowledge (EK): describe the knowledge required to perform the learning objective

AP Biology Science Practices include:

• Concept explanation

• Analyze visual representations

• Determine scientific questions and methods

• Represent and describe data

• Perform statistical tests and data analysis

• Develop and justify scientific arguments using evidence

• Scientific method: a step-by-step process used by scientists to investigate questions, gather evidence, and draw conclusions based on experiments and observations.

1. Make an Observation

• Ex: monarch butterflies can sometimes be found flying at night near very bright, artificial sources of light (like streetlights).

2. Ask Questions

• Can this be manipulated and turned into a repeatable experiment?

• Research the Topic:

  • Monarch butterflies are diurnal (active/flying during the day; resting/not flying at night)

  • Circadian rhythm (internal clock tracks day/night)

  • Migratory (use sunlight as a compass; antennae help track the sun during flight)

  • Proteins important for flight are processed at night while the butterfly rests

3. Form Hypotheses:

• Hypothesis: a testable explanation for an observation

  • A hypothesis must be testable!

• Explains the relationship between variables and what the researcher expects to happen to one variable if another variable changes

• “If … , then … (because…)” but it does not need to be in this format

  • “If”- the independent variable

  • “Then”- the dependent variable

  • “Because” - optional explanation

Results either support or do not support/refute the hypothesis

• NEVER SAY, “The hypothesis is correct”

Multiple Choice

A student notices that water striders can walk on the surface of a pond but sink when a small amount of soap is added to the water. She wants to investigate why this happens. According to the general steps of the scientific method, what should be her immediate next step after making this observation and asking a question?

• Conduct a controlled experiment with varying amounts of soap.

• Analyze data from similar experiments published in scientific journals.

• Formulate a testable hypothesis regarding the effect of soap on the surface tension of water.

• Draw a conclusion that soap disrupts hydrogen bonds between water molecules.

Multiple Choice

Which of the following represents a properly formulated hypothesis for an investigation into the effect of temperature on the rate of an enzyme-catalyzed reaction?

• Enzymes work best at 37C because that is human body temperature.

• If the temperature increases to 60C, then the rate of the reaction will decrease because the enzyme will denature.

• The rate of reaction is dependent on the temperature of the solution.

• Does increasing the temperature affect the amount of product produced?

Hypothesis tests: statistical test procedures to test hypotheses about a population based on a sample

• Helps to determine if observed differences between groups are statistically significant or due to chance

Null hypothesis (H₀)

• Hypothesis that there is no effect or no difference between the two groups of data

  • Observations are the result of chance

Alternative hypothesis (H_a: H₁, H₂…etc.)

• Hypothesis that there IS a relationship/ effect/ difference between the two groups of data

  • Observations are due to a nonrandom cause - not by chance

Why do we need a null for hypothesis testing?

• Provides a baseline, which we can use to compare our sample results to

• Allows for a hypothesis to be tested in a meaningful way using statistical tests.

  • Use the data to determine if there is strong enough evidence to reject the null hypothesis, meaning the alternative hypothesis may be supported by the data

When to use Statistical Tests

• Standard Deviation - Use this when you simply need to describe the variability or spread of your data set.

  • Low SD: Data points are close to the mean (consistent data).

  • High SD: Data points are spread out (inconsistent data).

  • You rarely use this to argue "significance" on its own. It is mostly a descriptive stat.

• Standard Error of the Mean (SEM) & Error Bars

  • Use this when you need to determine if two groups are statistically different from each other (e.g., Did the fertilizer actually make the plants grow taller, or was it just chance?).

  • You will calculate 2 x SEM (2 Standard Errors) to create 95% Confidence Interval error bars on a graph.

When to use Statistical Tests

• Chi-Square Goodness of Fit - Use this when you have categories (counts) and a theoretical prediction (like a Punnett Square or specific ratio).

  • Example: Genetics (3:1 ratio), Animal Behavior (do pillbugs prefer wet or dry sides?), Hardy-Weinberg predictions.

  • tells you if your real-world data ("Observed") matches your prediction ("Expected").

  • Interpreting the Result: X2 < Critical Value: Fail to reject the Null. (Data matches the prediction; differences are just chance). x2 > Critical Value: Reject the Null. (Data does NOT match the prediction; something else is going on).

Dropdown

Determine the kind of Hypothesis:

Does ivy grow better in areas with more sun versus in shady areas?

​

Select answer

- There is no difference in the growth of ivy between areas with more sun and less sun

​

Select answer

- Areas with more sun exposure have increased ivy growth

Multiple Choice

Researchers are testing a new fertilizer to see if it increases the growth rate of corn plants compared to a standard fertilizer.

Which of the following best represents the null hypothesis (H₀) for this experiment?

• The new fertilizer will result in a statistically significant increase in plant growth compared to the standard fertilizer.

• There will be no statistical difference in the growth rate of corn plants treated with the new fertilizer versus the standard fertilizer.

• If the new fertilizer is used, the plants will grow taller because of the added nitrogen.

• The new fertilizer will decrease plant growth due to nutrient burn.

Open Ended

Determine a suitable null hypothesis for the monarch butterfly experiment:

How does exposure to artificial light at night affect the number of monarch butterflies that initiate flight?

4. Performing Experiments: Groups

• Need at least 3 Groups

• Control Group: Used for comparison to the experimental group; help to validate statistical analysis and increase confidence in conclusions drawn from the experimental results; does not receive the independent variable (IV)

• Experimental Groups: Receives the experimental treatment (IV) being tested to observe its impact on the outcome (DV)

Kinds of Control Groups:

• Negative Control: Group NOT exposed to any treatment OR exposed to a treatment known to have NO effect

  • Helps to ensure there is NO effect when there should be NO effect

• Positive Control: Group not exposed to the IV but IS exposed to a treatment known to HAVE an expected effect

  • Ensures the experimental setup can produce a known effect; provides a reference point for what a known effect looks like

Example Negative Control

• A researcher wants to test the effect of caffeine on heart rate

  • Researcher will give negative control group a treatment that is known to have no effect on heart rate

• Water is known to have no effect on heart rate with consumption

  • If the water affects heart rate in the negative control group then there must be another variable affecting heart rate or the water is contaminated

Example Positive Control:

• A researcher wants to test the effect of a new antibiotic on a strain of bacteria

• How would the researcher know the new antibiotic (experimental group) is actually effective?

• Use an established antibiotic that is known to work (positive control group)

• If the experimental groups fail, but the positive control is successful, it is likely that the tested antibiotics are ineffective.

Dropdown

You want to design an experiment to determine if a new drug has an impact on lowering blood pressure. Test subjects with high blood pressure are given a new medication in pill form. The percentage of patients whose blood pressure decreased is measured.

A placebo pill that has no effect is a ​

Select answer

control.

A drug known to reduce blood pressure is a ​

Select answer

control.

Dropdown

You believe that when someone drinks a soda, their heart rate will increase. You set up an experiment, using various sodas, and various amounts of consumption and then measure the heart rate of the individuals.

An energy drink is a ​

Select answer

control.

Water is a ​

Select answer

control.

Dropdown

Proper soil composition is important for plants. Some elements, like nitrogen, are known to increase plant growth. A researcher wants to develop a soil with added minerals and nutrients to determine the best composition to promote healthy plant growth.

​

Select answer

control: soil with no added minerals or nutrients

​

Select answer

control: soil with nitrogen or fertilizer

Multiple Choice

A student is performing a Benedict’s test to detect the presence of reducing sugars in a food sample. They want to ensure their Benedict's reagent is working correctly and that their water source is not contaminated.

>Tube A: Contains distilled water + Benedict’s reagent.

>Tube B: Contains a glucose solution + Benedict’s reagent.

Which statement correctly identifies the controls?

• Tube A is the positive control; Tube B is the negative control.

• Tube A is the negative control; Tube B is the positive control.

• Both tubes are negative controls.

• Both tubes are positive controls.

Performing Experiments: Variables

• Independent: The one factor that is changed between groups; what is being manipulated; graphed on the x-axis

• Dependent: Factor that is measured and affected by the IV; graphed on the y-axis

• Constant: Factors kept consistent for all groups to ensure only the IV affects the outcome; aka controlled variables

Multiple Choice

Identify the dependent variable according to this graph.

• Seed Type

• Number of Bird Species

• Sunflower and Sorghum

• Sunflower

Dropdown

You are washing cars to earn money. For each car you wash, you earn $10.

The number of cars washed is the ​

Select answer

variable.

The amount of money made is the ​

Select answer

variable.

Dropdown

You are interested in how stress affects heart rate in humans. You place humans under different levels of physical stress and measure their heart rate

Heart rate is the ​

Select answer

variable.

Exposure to physical stress is the ​

Select answer

variable.

Dropdown

A scientist believes that fish can swim faster in warmer water versus in cooler water.

The temperature of the water is the ​

Select answer

variable.

How fast the fish can swim is the ​

Select answer

variable.

Multiple Choice

In an experiment testing the effect of pH on the function of the enzyme amylase, researchers ensure that the temperature, substrate concentration, and volume of the solution are identical in all test tubes.

These variables are best described as:

• Controls

• Dependent variables

• Constants

• Negative controls

Multiple Choice

A scientist is studying how the concentration of substrate affects the initial rate of a reaction catalyzed by the enzyme catalase.

When graphing the results of this experiment, how should the variables be plotted?

• Substrate concentration on the x-axis; Reaction rate on the y-axis.

• Reaction rate on the x-axis; Substrate concentration on the y-axis.

• Time on the x-axis; Substrate concentration on the y-axis.

• Enzyme concentration on the x-axis; Reaction rate on the y-axis.

PRACTICE QUESTIONS

Match

Match the scientific method

• Sensing the world, and noticing patterns and relationships

• This stage involves making an educated guess that includes a prediction,

• This phase involves a structured form of observation that allows you to examine one thing at a time.

• This last stage involves answering questions such as 1) Was the hypothesis correct? 2) Are there other lines of evidence that point in the same direction?

Making observations

Formulating hypothesis

Performing experiments

Drawing conclusions

• Making observations

• Formulating hypothesis

• Performing experiments

• Drawing conclusions

Multiple Choice

The alternative hypothesis usually states that there is no difference between the groups of data and, if there is a difference, it is due to chance.

• TRUE

• FALSE

Match

Match the following parts of the experiment: Sophia became interested in insulation while her parent’s new house was being built. She decided to determine which insulation transferred the least amount of heat. She filled each of 5 jars half-full with water. She sealed each jar with a plastic lid. Then she wrapped 4 of the jars with a different kind of insulation, leaving one jar without insulation. She put the jars outside in the direct sunlight. Later, she measured the temperature of the water in each jar.

• Control Group

• Experimental Group

• Independent Variable

• Dependent Variable

• Constant

Jar with no insulation.

Jars with insulation

Type of insulation

Temperature of water

How much water

• Jar with no insulation.

• Jars with insulation

• Type of insulation

• Temperature of water

• How much water

Multiple Choice

The __________________ variable is the variable you are observing and that changes in response to the _________________ variable.

• independent (experimental), dependent
• dependent, independent (experimental)
• dependent, control
• independent (experimental), control

Multiple Choice

Students put two plants in the sun.  

Plant A is in the sun for 10 minutes.  Plant B is in the sun for 1 hr.  

Students decide that plant B is better at photosynthesis.  This is wrong because...

• They didn't control all variables
• The independent variable was sunlight
• The dependent variable was amount of gas produced
• they used different amounts of water

Multiple Choice

Students are growing plants in the dark or in the light to see which grows better.  They put 3 seeds in the dark and 5 seeds in the light because they know seeds are more likely to dry out in the light. 
How could this experiment be improved?

• Put all plants in the light.
• Put all plants in the dark (where they sprout better)
• Put the same number of seeds in each container.
• put all the plants in the light so they grow better

Multiple Select

Which of these describes the control group? (Select two)

• The group that receives no independent variable or the normal version of the independent variable.

• The group that receives no dependent variable or the normal version of the dependent variable.

• The purpose is to provide a group to compare to the experimental groups.

• The purpose is to collect more data about the independent variable to increase validity.

Graphing
&
​Descriptive Statistics

​Graphing Requirements

• A good title (Y vs. X of _____)

• Label your axes with units, and use evenly spaced and scaled numbers to spread out the data.

• Clearly mark data points.

• When making a line graph, draw a line of best fit

• Any extrapolation beyond the last data point should be shown with a dashed line.

Bar Graphs

• Used for categorical independent variables (distinct groups like "Control," "Treatment A," "Treatment B").

• X-axis: Categories (Words/Labels, not continuous numbers).

  Y-axis: Dependent Variable (Numerical data).

• The Bar Height: Represents the MEAN (Average) of the data for that group.

• Error Bars must be included to show the variability of the data.

• Rule: If error bars overlap, there is likely no significant difference. If they do not overlap, the difference is likely statistically significant.

Box & Whisker Plots

• A data visualization that displays the distribution of data based on:

  • Minimum: lowest data point (excluding outliers).

  • Q1 : The median of the lower half of the data.

  • Median (Q2): The middle value (the line inside the box). Crucial Distinction: This is not the mean.

  • Q3 : The median of the upper half of the data.

  • Maximum: The highest data point (excluding outliers).

• Use to see the spread and skew of the data.

• Use to identify outliers (points that fall way outside the whiskers).

Line Graph

• Continuous Independent Variables.

• Used when the X-axis is numerical and continuous (e.g., Time, Temperature, pH, Concentration).

• Might have two y-axes for multiple scales - rarely have to graph these but often appear in the Multiple Choice section

• Plot the mean at each time point and include error bars (2SEM) to show significance changes over time

• Might have a log scale used for exponential data that spans several orders of magnitude, such as bacterial growth curves or DNA fragment sizes (semi-log plots).

• Most common reason to use a line graph in AP Bio is to calculate a rate (Change in Y / Change in X).

Data Tables

• Title must be descriptive (e.g., "Table 1: The Effect of [Independent Variable] on [Dependent Variable]").

• Units must be included in the headers (e.g., "Time (min)" or "Mass (g)"), not written next to every single number in the cells.

• Use Standard Convention: Independent Variable (IV) on the Left (First Column). Dependent Variable (DV) on the Right.

• Data in a column should have the same number of decimal places (sig figs) to show consistent precision.

• Use a Ruler - if the reader cannot tell which row a number belongs to, they cannot award points

Descriptive Statistics

• Calculate descriptive statistics for each group

  • control and experimental

• Mean = average of the data set

• Median = he middle value when data is ordered

  • Useful when data has outliers (extreme values) that might skew the mean

• Mode = the number that appears most frequently

• Range = the difference between smallest and largest values

  • This gives a quick sense of the spread but is sensitive to outliers.

• Standard Deviation = variability (spread) from the mean

• Standard Error of the Mean = measure of the precision of the sample mean

  • estimates how far your sample mean is likely to be from the true population mean.

Multiple Choice

Which of the following is a measure of variability?

• mean
• median
• mode
• standard deviation

Multiple Choice

The average of the data set

• Mean

• Median

• Mode

• Range

Fill in the Blanks

Multiple Choice

The difference between the largest and smallest values.

• Mean

• Range

• Standard Deviation

• Variability

Multiple Choice

The value that appears most often in a data set.

• Mean

• Median

• Mode

• Range

Standard Deviation

• Measures how spread out the data points are from the mean

• Low s= Data is clustered closely to the mean (Narrow Curve)

• High s = Data is spread far from the mean (Wide Curve)

• 68% of data is within 1 standard deviation

• 95% of data is within 2 standard deviations

• 99.7% of data is within 3 standard deviations

Multiple Choice

A measure of how spread out the data is from the mean.

• Variability

• Standard deviation

• Range

• Mean

Multiple Choice

All of the below sets have the same mean.
Set 1-Standard Deviation=3.1
Set 2-Standard Deviation=4.9
Set 3-Standard Deviation=1.7
Set 4-Standard Deviation=3.2
Which set of data probably has the points closest to the mean?

• 1
• 2
• 3
• 4

Labelling

Label the graphs as low and high standard deviation.

Drag labels to their correct position on the image

High

Low

Standard Error of the Mean

• How confident you are that your sample mean includes the population mean.

• Use +/- 2SEM error bars to create a 95% Confidence Interval - we are 95% confident the true population mean lies within this range.

• The larger the error bar, the less confident we are that the calculated mean is representative of the entire population.

• As sample size (n) gets larger, Standard Error gets smaller (your data becomes more precise).

• If the error bars of two groups overlap, there is no statistical difference between them (accept Null)

• If the error bars do not overlap, there is a statistically significant difference between the groups (fail to accept Null)

Multiple Choice

Used to determine the precision and confidence in the mean value and usually given as +/- 2 SE (How well does the mean sample represent the true mean of the population)

• Standard Error of the Mean

• Mean

• Standard Deviation

• Range

Multiple Choice

Low standard error equals _____ in confidence

• Decrease

• Increase

Multiple Choice

Standard error can be graphed as _____.

• Error Bars

• Bar Graph

• Standard Deviation Bars

• Line Graph

Multiple Choice

If error bars overlap, the difference _____ significant.

• is Not

• May Be

Multiple Choice

If error bars do not overlap, the difference _____ significant.

• Not

• May Be

Multiple Choice

True or false: Oranges are more expensive compared to apples.

• True, the average is higher (the bar graph)

• True, the error bars overlap, therefore, there is a significant difference.

• False, the error bars overlap, therefore there is NO significant difference. The variation is due to chance.

Fill in the Blanks

Multiple Choice

The differences in the graph:

• are not significant

• are significant

Multiple Choice

Which of the following is a correct conclusion to draw from this graph?

• Exercise increases blood plasma prolactin

• Exercise has a significant impact on blood plasma prolactin

• Exercise has no significant impact on blood plasma prolactin

Multiple Choice

Which group had a significant decrease in reaction time after the treatment?

• A

• B

• A & B

• C

Chi-square Goodness of Fit (GOF) = Observed vs Expected

• Use when you are comparing one (observed) data set to predicted (expected) values.

• The coin flip example.

• 1:2:1 ratios of progeny – think Punnett squares!

• Evaluating whether organisms are attracted or repelled by something in their environment.

• Does this population meet the predictions of the Hardy-Weinberg equilibrium?

  • Null: There is no difference between the expected and observed data.

  • Alternate: There is a difference between the expected and observed data.

• Assume p=0.05

• Reject Null Hypothesis if X2 > Critical Value

• Fail to reject the Null Hypothesis if X2 is < Critical Value

Multiple Choice

Degrees of freedom is calculated by

• dividing the standard deviation by the square root of the sample size

• subtracting one from the sample size

• adding together all values and dividing by the sample size

• squaring the difference between the observed and expected values

Multiple Choice

IF I flip a coin 250 times and I only get 107 tails, can I accept my null hypothesis that nothing is wrong with my coin - all due to chance?

• Fail to Reject the null hypothesis, the Chi Square value is greater than the critical value

• Reject the null hypothesis, the Chi Square value is greater than the critical value

• Fail to Reject the null hypothesis, the Chi Square value is less than the critical value

• Reject the null hypothesis, the Chi Square value is less than the critical value

​Practice Questions

Fill in the Blanks

Multiple Choice

The data sets show the test scores of a group for the last two tests.

Test 1: {75, 75, 85, 80, 65, 70, 65} Test 2: {95, 85, 85, 90, 90, 95, 100}

Which data set had the SMALLER standard deviation?

• Test 1 with a standard deviation of 6.9

• Test 2 with a standard deviation of 6.9

• Test 1 with a standard deviation of 5.2

• Test 2 with a standard deviation of 5.6