Shelly's study compared the effects of two different types of phonemic instruction on young children who volunteered to participate in the study. She gave each participant who completed the study two pencils and three stickers. These incentives are likely a strategy to decrease the threat of ___.

Differential Selection

Explanation: Differential Selection this is a selection threat with greatest concern when subjects cannot be randomly assigned to a treatment group, particularly if the groups had being unequal when the relevant variables were being considered before treatment intervention. By giving each participant pencils and stickers she is decreasing the threat of differential selection.

permafrost layer, it exists beneath a layer of soil, rock or sediment.