There are some internal threats to the core validity of Single-Case Research design. One of them is the testing thereat, which asserts that exposure tests can affect scores on succeeding exposures to that test (Kratochwill et al., 2010; Smith, 2012). An example of this is that when people are given the same test three times, they may perform better, which may provide a wrong impression of their ability. The second one is selection threat. A context here is a case in which there is a study on why some societies subjugate women and the only participants selected to act as respondents are male. The third one is history. A good example is the occurrence of events that might change someone opinion about a subject of study during the period.
In that vein, the other aspect is the statistical regression. A pattern of this is when there is a study on what makes students pass an examination, and only bright students are selected from a class of mixed abilities to act as the respondent. Attrition is another problem. It relates to the loss of respondents is research. For example, if there is a study on the perception of the concept of men's masculinity and the participants are men and women at the beginning, if either of the sexes dropouts of the exercise, the resulting data cannot be reliable. An additional element is instrumentation in which, for example, if students are to be tested on their knowledge of technology, they may likely score better if relative simple machines such as phones are used to prove that than when computers are used.
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Additive and interactive effects is another one. A case in point here is where there the threat of having a repeated test to assess the academic ability of learners can be replaced with different assignments. Another example is the issues of setting or side effects in which for example, a threat may exist when a single subject such a classroom to come up with a conclusion yet it entails other internal variables in terms of groups of people as indicated in Smith (2012. Another case is the lack of constant of a standard of measuring variables that result in inconsistency. For example, in measuring the performance of learners for one month, the failure to include all outcomes over the whole period may result in invalidity.
References
Kratochwill, T. R., Hitchcock, J., Horner, R. H., Levin, J. R., Odom, S. L., Rindskopf, D. M. & Shadish, W. R. (2010). Single-case designs technical documentation. Retrieved from What Works Clearinghouse website: http://www.eric.ed.gov/contentdelivery/servlet/ERICServlet?accno=ED510743. 2)
Smith, J. D. (2012). Single case experimental designs: A systematic review of published research and current standards. Psychological Methods, 17, 510-550
