Gathering data--Experiment

Observational studies and experiments

In an observational study, we measure or survey members of a sample without trying to affect them. For example, A study took random sample of adults and asked them about their bedtime habits. The data showed that people who drank a cup of tea before bedtime were more likely to go to sleep earlier than those who didn't drink tea. This is an observational study. Generally, people can't draw conclusions only through an observational study, like we can't say for sure that tea is the factor that cause the sleeping time to change, it might be the food people eat or something else that cause the differences. In a word, an observational study does not prove a hypothesis.

In a controlled experiment, we randomly assign people or things to groups. One group receives a treatment and the other group does not.For example, Another study took a group of adults and randomly divided them into two groups. One group was told to drink tea every night for a week, while the other group was told not to drink tea that week. Researchers then compared when each group fell asleep.This is an experiment, because the person who conducted it set up two groups--a experimental group and a control group, which enables us to actually test wether or not tea is the factor that cause the differences in sleeping time.

Language of experiments

EXAMPLE Karina wants to determine if kale consumption has an effect on blood pressure. She recruits 100 households and randomly assigns each household to either a kale-free diet plan or a kale-based diet. At the end of two months, she plans to compare the original and final blood pressures for members of each household.

1.What is the explanatory variable?An explanatory variable explains changes in another variables. Karina is curious if a kale-based diet will cause changes in blood pressure. Therefore, the explanatory variable is kale consumption.

2.What is the response variable?

A response variable measures the result of a study. Karina is measuring the change in blood pressure at the end of the study. Therefore, the response variable is the change in blood pressure.

3.What are the treatments?

A treatment is the specific thing given to individuals in an experiment. Karina is giving some households a kale-based diet and other households a kale-free diet. Therefore, the treatments are the kale-based and kale-free diets.

4.Who or what are the experimental units?

An experimental unit is who or what we are assigning to a treatment. Karina is randomly assigning each household to a treatment, not individual members. Therefore, the households are the experimental units.