I wonder if you could provide me descrpitions or explanations on questions below:

In addition to measurement error, something else is also creating variability in the distribution of 35 scores (i.e., head circumferences). Besides measurement error, what is another reason for the variability in the above distribution of 35 infants' head circumferences?

1. Treatment variability; a treatment created this variability.

2. Individual differences; some infants have larger heads than others.

In the previous question, the variability in head circumferences was created by both measurement error, which is always present, and the fact that the 35 infants' heads varied in size, called individual differences variability. In which of the following distributions of scores would there be more variability created by individual differences?

1. The head circumferences of 50 first graders.

2. The head circumferences of 50 elementary school children (first through fifth graders).

2. In many research situations, researchers generate variability with different treatment conditions. For example, if you thought physically touching prematurely born infants would increase their growth, you could conduct a study with two samples of prematurely born infants. Infants in Group 1 could be touched with skin-to-skin contact for 6 hours a day. Infants in Group 2 could be touched only by someone wearing gloves. After 4 weeks of these differing treatment conditions, you could compare the head circumferences of the babies in these different conditions. In this study, there are three things creating variability in infant's head circumference. The fact that measuring an infant's head circumference is hard to do accurately contributes to the amount of ________________ in this study.

1. treatment difference validity

2. individual difference validity

3. measurement error validity