1.4.2 - Causal Conclusions

In order to control for confounding variables, participants can be randomly assigned to different levels of the explanatory variable. This act of randomly assigning cases to different levels of the explanatory variable is known as randomization. An experiment that involves randomization may be referred to as a randomized experiment or randomized comparative experiment. By randomly assigning cases to different conditions, a causal conclusion can be made; in other words, we can say that differences in the response variable are caused by differences in the explanatory variable. Without randomization, an association can be noted, but a causal conclusion cannot be made.

Note that randomization and random sampling are different concepts. Randomization refers to the random assignment of experimental units to different conditions (e.g., different treatment groups). Random sampling refers to probability-based methods for selecting a sample from a population.

The act of randomly assigning cases to different levels of the explanatory variable
Changes in one variable can be attributed to changes in a second variable
A relationship between variables

Example: Fitness Programs Section

Two teams have designed research studies to compare the weight loss of participants in two different fitness programs. Each team used a different research study design.

The first team surveyed people who already participate in each program. This is an observational study, which means there is no randomization. Each group is comprised of participants who made the personal decision to engaged in that fitness program. With this research study design, the researchers can only determine whether or not there is an association between the fitness program and participants' weight loss. A causal conclusion cannot be made because there may be confounding variables. The people in the two groups may be different in some key ways. For example, if the cost of the two programs is different, the two groups may differ in terms of their finances. 

The second team of researchers obtained a sample of participants and randomly assigned half to participate in the first fitness program and half to participate in the second fitness program. They measured each participants' weight twice: both at the beginning and end of their study. This is a randomized experiment because the researchers randomly assigned each participant to one of the two programs. Because participants were randomly assigned to groups, the groups should be balanced in terms of any confounding variables and a causal conclusion may be drawn from this study.