Final answer:
Within-treatments variance in the context of treatment for arachnophobia is primarily contributed to by individual differences in the severity of arachnophobia. Systematic differences due to treatment efficacy tend to be the concern of between-treatments variance. Individual differences are a common and expected source of within-treatments variance.
Step-by-step explanation:
When evaluating which factors might contribute to within-treatments variance, it's crucial to understand that variance refers to the degree of variation or differences observed in a set of data. Within-treatments variance specifically looks at differences that occur among individuals experiencing the same treatment condition. In the context of psychological treatment for a condition such as arachnophobia, here are the contributions:
- Individual differences in the severity of arachnophobia certainly contribute to within-treatment variance because they represent variations in baseline or pre-treatment conditions of the individuals. These differences could affect how each person reacts to the same treatment.
- Differences caused by the effectiveness of different treatment modalities (in vivo vs. imaginal flooding) would be considered systematic differences and thus, typically relate more to between-treatments variance rather than within-treatments variance.
- Differences that are attributed to the varying efficacy of methodologies used for the treatment of arachnophobia, if not properly randomized or controlled, could also contribute to within-treatments variance unintentionally.
To illustrate using an unrelated example mentioned in the information provided, if both injured and non-injured salamanders produce chemical compounds that cause avoidance behavior in non-injured salamanders, this is an individual difference contributing to within-treatments variance in a study of salamander behavior.