McDonalds Case

A threat to external validity is an explanation of how you might be wrong in making an assumption. Where Morgan Spurlock went wrong was assuming that a person would eat at McDonalds for three meals a day. Clearly no one does that. At the most someone might eat at McDonalds for lunch and dinner for three days out of the week. I can't possible see anyone other than Morgan, eating three meals a day at McDonalds for more than thirty days.

Internal validity addresses the "true" causes of the outcomes that you observed in your study. Strong internal validity means that you not only have reliable measures of your independent and dependent variables BUT a strong justification that causally links your independent variables to your dependent variables. At the same time, you are able to rule out extraneous variables, or alternative, often unanticipated, causes for your dependent variables. Thus string internal validity refers to the unambiguous assignment of causes to effects. Internal validity is about causal controlThreats to Internal Validity

a. Selection

Subjects bring with them into the investigation unique characteristics, some learned and some inherent. Examples include sex, height, weight, color, attitude, personality, motor ability, and mental ability. If assigning subjects to comparison groups results in unequal distribution of these subject-related variables, then there is a possible threat to internal validity. Suppose that subjects in two comparison groups are unlike with respect to the independent variable and one of these subject-related variables. If scores on the dependent measure differ between the groups, the discrepancy may be due to the independent variable or to the subject-related variable.

b. History

Outside events may influence subjects in the course of the experiment or between repeated measures of the dependent variable. Suppose that the dependent variable is measured twice for a group of subjects, once at Time A and later at Time B, and that the independent variable is