Also included are operational variables of BMI and waist circumference as well as physical activity and use of high energy foods (Pietilainany, Kapro, Rasomen, 2001).
The variables that were operationalized were BMI, waist circumference, high energy food intake. They set an average BMI over the years of study per age group as well as waist circumference. The last studies of these groups of children were done when they reached the ages of 22-27 years old. Waist circumferences and high energy food intake were also defined along this continuum of study.
One of the validity tests that they had to use here was the statistical conclusion validity and the reason for that is because there are many different variables being used. The most important variable to look at here is the use of twin to measure genetic effects on activity and obesity. All of the genetic backgrounds were traced and these twins did meet all of the criteria for a valid study showing that the statistical results were appropriate given the causal nature of the genetic part of the study (Burns & Grove, 2007).
Internal validity is about whether this is real world testing and in this case it certainly is as there are definitely real world issues for twins like any other child as far as developing or not developing obesity. Extraneous validity is important also and these children were allowed to live their normal lives with each of the parameters that might affect them tracked and then checked again over the years. In other words it was not set up that one twin would be sedentary while the other was active leading to one that was obese and one that was not. They lived their usual lives and if one was obese, it was determined by the real lifestyle that child led.
Mustelin L, Silventoinen K, Pietalainen K. (2009) Physical activity reduces the influence of genetic effects on BMI