This question asks to identify the factors that negatively impact the internal validity of an experimental research study. Internal validity refers to the degree to which a study can rule out alternative explanations for the observed outcomes, ensuring that the relationship between the independent variable (the treatment) and the dependent variable (the outcome) is truly causal.

Understanding Internal Validity Threats

Several factors can compromise internal validity, making it difficult to determine if the experimental manipulation caused the observed effects. Let's examine the options provided:

Analysis of Factors Affecting Internal Validity

• B. Instrument Decay: This occurs when the measurement instrument or the way it's used changes over the course of the study. For example, if researchers become more lenient or strict in scoring over time, or if a measuring device degrades. This inconsistency means observed changes might be due to the instrument itself rather than the treatment, directly threatening internal validity.
• C. Contemporary events: Also known as the 'History' threat. This involves external events that happen between the pre-test and post-test measurements, which could independently influence the dependent variable. If such an event affects participants in one condition but not the other, or affects all participants differently, it confounds the results and undermines internal validity.
• E. Bias in selecting subjects in sample: This refers to 'Selection Bias'. If the groups involved in the experiment (e.g., treatment group and control group) are not equivalent at the start due to systematic differences in how participants were selected or assigned, any observed differences at the end might stem from these initial differences rather than the treatment itself. This is a critical threat to establishing causality.
• A. Placebo effect: While the placebo effect (participants responding due to their belief in the treatment) can influence outcomes and is often a concern in experimental design (requiring controls like placebo groups), it's sometimes viewed as a specific type of participant bias or response rather than a direct structural threat like history or selection bias in the same category as B, C, and E.
• D. Hawthorne effect: This is the effect where participants modify their behavior simply because they are aware of being observed. Like the placebo effect, it represents a form of reactivity that can influence results, but B, C, and E are generally considered more fundamental threats to the core logic of experimental control and establishing cause-and-effect.

Conclusion on Internal Validity Factors

Based on the standard threats to internal validity in experimental research, Instrument Decay (B), Contemporary events (C), and Bias in selecting subjects (E) are direct and significant factors that can distort the findings and weaken causal conclusions. These issues relate to the consistency of measurement, the influence of external factors, and the initial comparability of experimental groups.