Experimental Design W/S #2
For each of the following: A. Tell whether you think the experiment described is a controlled experiment.
B. If it is not, tell how you would change it to make it a controlled experiment
C. If it is, tell what independent and dependent variables are being studied.
1. McDonalds restaurants want to know if their new banana milkshake is popular enough to add it to their
regular menu. They count the number of banana shakes sold at 20 different restaurants over a 2 week
period to decide. This is NOT a controlled experiment because it lacks a control. There is nothing to
which they can compare the 20 banana shakes sold. They would need the number of shakes sold that are
NOT banana (i.e. the total number of all other flavors sold) to make a comparison. If they sold 20
banana shakes versus only 5 of all other flavors, then the banana shake is a hit. If they only sold 20
banana shakes versus 500 total of all other flavors, then banana is probably not a flavor they would want
to continue.
2. A consumer advocate group wants to test whether a fertilizer company’s claim that plants grow taller
with their fertilizer is accurate. They perform the following experiment: Forty coleus plants are grown
from clippings of a single plant. All of the plants are approximately the same height at the start of the
experiment. Twenty of the plants receive the recommended amount of fertilizer with each watering. The
remaining twenty plants receive the same amount of water with no fertilizer. All forty plants are grown
at 25 ˚C with 10 hours of light daily. The height of each plant is recorded every 5 days for 2 months.
This one is a controlled experiment. The presence of fertilizer is the independent variable. The height of
the plants is the independent variable.
3. An MIT graduate student wants to know whether posting nutritional information near foods in a
supermarket will change people’s buying habits. The supermarket she chooses has records of dairy
product sales for the previous 3 months. She posts signs telling the nutritional benefits of dairy products
near the dairy section for the next three months and again keeps records of dairy product sales. This one
is a tough call. She tried to have a control (the 3 months previous sales) to which to compare her
experimental results. However, there are other variables that may not have been standardized, so the
presence of nutritional information (her independent variable) may not be the only thing that is different
between the two sets of data. For example, the control data may have been recorded during December,
January, and February when the weather is cold and people may not feel like eating dairy products (most
of which are cold). If her test data was then taken during March, April, and May when the weather was
getting warm, an increase in sales may really be due to the change in the weather and not due to her
signs. Experiments involving human behavior are generally difficult to control and require extra care in
design. Perhaps she could have used several stores with similar demographics during the same 3 month
period, but placed signs in half of the stores while leaving the other half without signs.
For each of the questions below fill in the chart to identify the dependent and independent variables and to
classify each as continuous or discrete.
Question
Mary wants to find out whether plants
produce more leaves when grown in red,
blue, or green light.
Tom wonders whether the speed of his
model train affects the amount of time it
takes the train to stop.
Christine is studying the pressure in a
closed container at different temperatures.
John is trying to determine whether
basketball players have lower blood
pressure than people who don’t play
basketball.
Janet is doing an experiment to see if the
number of peanuts produced per plant
changes when the amount of fertilizer
given to the plants is changed.
Teresa wants to see if different brands of
yeast make her loaves of bread rise to
different heights.
Independent
variable
Continuous
or discrete
Dependent
variable
Continuous
or discrete
Color of light
Discrete
Average # of
leaves
Continuous
Speed of
train
Continuous
Stopping time
Continuous
Temperature
Continuous
Pressure
Continuous
Basketball
players vs.
nonplayers
Discrete
Blood
pressure
Continuous
Amount of
fertilizer
Continuous
# peanuts
produced per
plant
Continuous
Brand of
yeast
Discrete
Height of
bread
Continuous