Introduction to Experiments (College Board AP® Statistics)
Study Guide
Written by: Mark Curtis
Reviewed by: Dan Finlay
Introduction to experiments
What is an experiment?
An experiment is a type of study where certain conditions (treatments) are applied to items or individuals (experimental units) to see if it causes a response
When experimental units are people they can also be called subjects
For example, if 10 swimmers from a club are given a new type of swim cap to see if it improves the time it takes to swim one lap, then:
an experimental unit is 'a swimmer from the club'
the treatment is 'being given a new type of swim cap'
Examiner Tips and Tricks
You will be expected to identify experimental units and treatments in the exam.
What are explanatory and response variables in experiments?
The explanatory variable is the quantity or property that you want to change to see if it causes a response
The treatment is a change in this explanatory variable
(or sometimes the combined change of multiple explanatory variables)
The response variable is the response shown by the experimental units after the treatment has been applied
This can often be measured on a scale
For the swimming example above:
the explanatory variable is 'the type of swim hat'
the response variable is 'the time it takes to swim one lap'
They key question in an experiment is
"Does a change in the explanatory variable cause a change in the response variable?
Examiner Tips and Tricks
When asked to identify the response variable, it is advisable to copy the exact wording from the question, including any scale (this can be quite a long sentence!)
What is an observational study?
An observational study is a way of finding out information from a sample by simply recording what you observe naturally
e.g. filling in a tally chart or a sending out a questionnaire (sample survey)
They can be either
retrospective, where data is collected then analyzed
e.g. sample surveys
or prospective, where a sample is observed for a period of time into the future
e.g. following the behavior of a class of students over two years
How is an experiment different to an observational study?
In an observational study, you do not influence or change the individuals
You simply observe them
However, in experiments you do influence the individuals because you impose a treatment on them to see if it causes a response
Worked Example
A fruit seller wanted to know if washing fruit made it last longer. A random sample of 30 strawberries was taken and 15 were washed and the other 15 were left unwashed.
The time, in hours, that it took for the strawberries to decay was then measured.
(a) Describe the experimental units.
Answer:
The experimental units are the 30 strawberries
(b) Describe the treatments.
Answer:
The two treatments are washing 15 strawberries and leaving the other 15 strawberries unwashed
(c) Describe the response variable.
Answer:
The response variable is the length of time, in hours, that it takes for a strawberry to decay
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