This article discusses the threats to validity (internal and external) irrespective of the approach. In other words, the "n" justifies the means. Threats to validity include: Selection--groups selected may actually be disparate prior to any treatment.. Mortality--the differences between O 1 and O 2 may be because of the drop-out rate of subjects from a specific experimental group, which would cause the groups to be unequal.. Others--Interaction of selection and maturation and interaction of selection and the experimental variable. Subject Characteristics• Four of many possible characteristics a. Severity of disability b. Socioeconomic level of parents c. Physical strength and coordination d. … ): 5. EVALUATING THREATS TO INTERNAL VALIDITY• Follows a procedure similar to the experimental research.1. Timeline: Time is of paramount importance in research. Statistical regression 6. Behavioral observations: becoming more/less strict or changing criteria Results could be due to changes in the instrument and not the treatment Threats to Internal Validity Internal validity threats. History ; An event that occurs during the intervention that could have an impact on the results ; Maturation ; Bias from biological, natural, or social events that can bias results; 6 Internal validity threats. 4 Statistical Inference The larger the sample size (n) the more confident you can be that your sample mean is a good representation of the population mean. Attrition (experimental mortality) 14 . Must have good internal validity to have good external validity, but doesn’t guarantee good external validity. Threats to Internal Validity in Causal-Comparative Research • Subject Characteristics • The possibility exists that the groups are not equivalent on one or more important variables • One way to control for an extraneous variable is to match subjects from the comparison groups on that variable Threats to Internal Validity II • Maturation • Relates to participants growing older, wiser, stronger, etc. The following must be controlled to reduce threats to internal validity. Research reactivity 7. A threat to internal validity is the issue of accurate and confident interpretation of its results that is at the center of the discussion of the validity according to (“Threats to internal and external validity – SlideShare.,” n.d.). Threats to Internal and External Validity 1. • Testing • Problems resulting from repeated measurements • Practice effect • Memory of certain items Factors that threaten the validity of research findings Material for this presentation has been taken from the seminal article by Don Campbell and Julian Stanley: Experimental and quasi-experimental designs for research on teaching, which was first published as Chapter 5 in N.L Page (1963), Ed., Handbook of Research on Teaching. What’s common across these examples of threats to internal validity is that they correspond to unmeasured differences between the two groups » Imagine a latent fixed effect across the two groups – This results from non-random assignment to treatment and control » This will confound any comparison between the two groups Threats to internal validity. Representativeness of sample, setting and procedures ! 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