Final answer:
Pre-experimental designs are less rigorous than true experimental designs because they often lack random assignment and control groups, which reduces the ability to isolate the experimental manipulation's effect and make strong causal inferences.
Step-by-step explanation:
Pre-experimental designs differ from the classical or true experimental designs in terms of rigor and control over variables. In a true experimental design, participants are randomly assigned to either the control or the experimental group, and the only difference between the two groups is the level of the independent variable applied. The experimental group receives the experimental manipulation, while the control group does not. In pre-experimental designs, this strict random assignment may be absent, potentially allowing for more confounding variables to affect the outcome, which reduces the ability to make strong causal inferences.
For instance, in a classical experimental design, a control group might receive a placebo while the experimental group receives the actual treatment, allowing for a comparison of effects due to the treatment alone. Pre-experimental designs may lack such control groups or random assignments, resulting in less reliable data and weaker evidence for causality.