Which threat to internal validity is most directly reduced by random assignment?

Which threat to internal validity is most directly reduced by random assignment?

• A. Attrition bias due to loss of participants.
• B. Instrumentation drift across measurements.
• C. Confounding due to pre-existing differences between groups.
• D. Demand characteristics of participants.

Answer: (C)

Random assignment directly targets confounding from pre-existing differences between groups. By randomly assigning participants to conditions, the groups become, on average, similar in both measured and unmeasured characteristics before the intervention. This means that any observed effect is more likely due to the experimental manipulation itself rather than pre-existing differences, which is the core threat to internal validity that randomization mitigating. Attrition bias from differential dropout isn’t guaranteed to be prevented by random assignment, though it can be mitigated with good retention efforts and analysis strategies. Instrumentation drift across measurements relates to changes in measurement tools or procedures over time, which isn’t solved by how participants are allocated to groups. Demand characteristics stem from participants’ guesses about the study aims and can influence behavior regardless of assignment; randomization doesn’t inherently fix that.

Random assignment directly targets confounding from pre-existing differences between groups. By randomly assigning participants to conditions, the groups become, on average, similar in both measured and unmeasured characteristics before the intervention. This means that any observed effect is more likely due to the experimental manipulation itself rather than pre-existing differences, which is the core threat to internal validity that randomization mitigating.

Attrition bias from differential dropout isn’t guaranteed to be prevented by random assignment, though it can be mitigated with good retention efforts and analysis strategies. Instrumentation drift across measurements relates to changes in measurement tools or procedures over time, which isn’t solved by how participants are allocated to groups. Demand characteristics stem from participants’ guesses about the study aims and can influence behavior regardless of assignment; randomization doesn’t inherently fix that.