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Table 1 Categories of reasons why attempts to replicate a research finding may be unsuccessful

From: Improving the reproducibility and integrity of research: what can different stakeholders contribute?

Category 1 A valid research claim was made based on the observed data, but the statistical test had returned a Type I or “false positive” error
Category 2 The claim that was made was valid under the particular circumstances under which it was tested but is not observed under the circumstance in which replication was attempted. These different circumstances may be obvious or subtle, and their impact on the observed phenomena may or may not be important in understanding the question at hand
Category 3 The observations may have been due to sub-optimal study designs (which for instance allow the emergence of experimenter bias, or selective data presentation, or hypothesising after results are known), which might generally be considered as questionable research practices, with varying degrees of researcher culpability
Category 4 The research claim may have been made following deliberate researcher malfeasance such as falsification or fabrication