Constants vs. Control Groups

Constants vs. Control Groups

What are they?

Most people mix these up because...

• They sound the same

• They both have to do with experiments

• They are closely related, but not identical

• Scientists insist on calling constants “controlled variables” sometimes, which makes it even more confusing

Multiple Choice

Quick Review: An independent variable in an experiment is the thing we...

• Measure

• Change

• Keep the Same

• Build

Multiple Choice

A dependent variable is the thing we...

• Measure

• Change

• Keep the Same

• Observe

Let’s Start with Constants

• Controlled S.M. experiments = cause & effect

• How does [independent variable] affect [dependent variable]?

• In a controlled experiment, we can only ever change one thing at a time. This is our independent or manipulated variable

• Ex: How does sunlight affect plant growth?

• I can ONLY change the amount of sun each plant gets and NOTHING else, or my experiment isn’t valid!

Constants

• Because we can only change one thing, every other part of our experiment must be kept the same.

• Constants = the things we keep the same in an experiment so that we only change ONE thing [I.V.]

Multiple Choice

How many things can we change in an experiment?

• None

• Two

• One

• Three

• As many as we want

Without constants, we get error!

• Was it the independent variable or something else that caused the outcome of our experiment? 

• If we allow more than one thing to be changed, we’ll never know!

• Error = something besides our I.V. affecting our experiment results

In other words... Good scientists are control freaks!

Open Ended

Review: Why are constants important in a controlled experiment?

Example of Constants

• My testable science question:

• Does the temperature outside affect how high my kite will fly?

Multiple Choice

What's the independent variable: Does the temperature outside affect how high my kite will fly?

• The kite

• How high the kite will fly

• The temperature outside

• The wind speed

I.V. = The temperature outside

This is the ONLY thing I can change in my experiment!!!

Multiple Choice

What's the dependent variable: Does the temperature outside affect how high my kite will fly?

• The kite

• How high the kite will fly

• The temperature outside

• The wind speed

As a control-freak scientist, I have to ask myself: what else besides the independent variable could affect my dependent variable, even accidentally?

Open Ended

Think about flying a kite: what are some things that could effect how a kite flies BESIDES the temperature outside? List as many as you can think of!

These are constants, or things we need to keep the same during our experiment.

We can ONLY change the temperature outside, so wind speeds, geographic location (where I’m standing), which kite I use, what time of day it is, the humidity/ moisture in the air, birds running into my kite, whether other people are also flying kites, etc. all need to STAY THE SAME!!!

Multiple Select

If we don't pay attention to constants in our experiment, what could happen? Check all that apply.

• We could have error

• More than one thing could affect the dependent variable

• We won't know if our results are truly based on the independent variable or something else

• Our results will be inaccurate

• Nothing will happen

It’s not easy!

Some things in experiments may be outside of our ability to control! We do our best and keep careful notes of any outside factors that may have influenced our results.

Another Example

• My testable science question:

• Which brand of gum, Hubba Bubba or Double Bubble, results in bigger bubbles blown?

The brand of gum is the independent variable.

This is the ONLY thing we can change!!!

Multiple Choice

Which brand of gum, Hubba Bubba or Double Bubble, results in bigger bubbles blown? What's our dependent variable?

• Brand of gum

• Color of gum

• Size of bubble blown

• Time we chew the gum for

Open Ended

Which brand of gum, Hubba Bubba or Double Bubble, results in bigger bubbles blown? What do we need to keep constant here so that we ONLY change the brand of gum? What could (even accidentally) impact our results in this experiment? List as many things as you can!

Some possible constants

Person blowing the bubble, length of time gum is chewed before blowing the bubble, the temperature in the room, the moisture in the air, how I measure the size of each bubble, etc.

Control & Experimental Groups

Control & Experimental Groups

• When we design a controlled experiment, we want to see how one independent variable affects a dependent variable.

• (How does the air temperature affect how fast ice melts?)

We only want to measure one IV, so we brainstorm all the constants that could affect our experiment (even accidentally).

• Location of ice, surface the ice is sitting on/ in, size of the ice, airflow in the room, temperature of the container when we put the ice in, the timer we use, the person doing the timing, the thermometer we use, the person holding/ reading the thermometer, etc.

To make sure we didn’t miss anything, though, we will use a control group.

Experimental Group

• An experimental group is whatever you’re experimenting on; gets the I.V. 

• In our ice example, the ice that we’re exposing to different air temperatures

Control Group

• A control group is another group of whatever you’re experimenting on, and is identical to the experimental group EXCEPT: you do NOT change the independent variable!

• The control group gets all the same constants as the experimental group

• Constants in our ice example, same container, same airflow, same room, timer, thermometer, etc.

Control Group, Pt. 2

• Instead of changing the independent variable for the control group, you make the I.V. another constant OR do not give the control group the I.V.

• In our ice example, since our independent variable is air temperature, our control group needs to have one constant air temperature, as well as ALL the same constants as the experimental group.

Multiple Select

What are the two groups in a controlled experiment called?

• Control Group

• Change Group

• Experimental Group

• Observe Group

Multiple Choice

The group that does receive the independent variable change is the

• Control Group

• Experimental Group

Multiple Choice

What do the experimental and control groups in an experiment NOT have in common?

• They use the same materials

• The dependent variable (what we're measuring) is the same for both groups

• The independent variable (what we're changing) is the same for both groups

• The constants (except the I.V.) are the same for both groups

• We collect the same data for both groups

Control Group Example

• My testable science question:

• What effect does the amount of water have on a plant’s growth?

Multiple Choice

What's the independent variable: What effect does the amount of water have on a plant’s growth?

• the amount of water

• the plant's growth/ height

Multiple Choice

What's the dependent variable here: What effect does the amount of water have on a plant’s growth?

• the amount of water

• the plant's growth/ height

Open Ended

What effect does the amount of water have on a plant’s growth? What are constants I need to pay attention to in this experiment? List as many as you can!

Some constants

• Type/ species of plant used, ruler I measure with, temperature of the room, amount of sunlight, exposure to insects, pots/ containers, potting soil/ fertilizer, etc.

Experimental Group

• Gets the independent variable (I change the amount of water) and all the constants

Control Group

• Gets all the constants PLUS the I.V. becomes a constant (each plant gets the same amount of water) 

My Experimental Design

My CG should have the same results as WHICH plant in my EG?

Multiple Choice

My control group plants should show the same results as which plant in my experimental group?

• Plant 1

• Plant 2

• Plant 3

My Experiment Results

CG as expected; data looks accurate!

Something is wrong... CG inconsistent!

Either...

• I missed a constant and something else impacted the plants that wasn’t just water - error!

• Or, there was just something wrong with those particular plants, but….

Either way, I know I need to reflect on possible causes of this outcome (constants I may not have considered before), and then redesign and repeat my experiment.

Open Ended

I just finished an experiment, and the three test subjects in my control group are all showing different results from each other. What does this tell me about my experiment?

Constants

• Controlled Experiments = How does [independent variable] affect [dependent variable]?

• We only change ONE thing in a controlled experiment - only ONE independent variable

• Constants are all the things/ conditions we keep the same in an experiment so the ONE I.V. is the only thing that’s changed

Control Groups

• 2 groups in an experiment: experimental group & control group

• The experimental group IS manipulated/ gets the independent variable

• Control groups get all the same constants as the experimental group, PLUS they DON’T get the I.V. or the I.V. becomes a constant.