Researchers must be aware of the possible biases when using physical traces, including problems such as the selective deposit or the selective survival of traces. Spurious relationships exist when data falsely indicate that two variables are associated, when in fact they are not directly associated, can occur when taking a too simplistic approach to the hypothesis testing. Using a multimethod approach to data collection can assist in ruling out alternative explanations. Coding requires that relevant descriptive categories and appropriate units be defined and measured. When combined with other measures of behaviour, the researcher can often increase the study's internal validity and reduces the chance that the results are due to some artefact of the measurement process.
Case study The case study is an intensive description and analysis of a single individual.
It is used primarily in clinical or counselling psychology. It presents the data that would be gathered in a clinical interview, perhaps including tests and archival records, and describes the outcome of the selected treatment program. While the efficacy of the treatment is usually the central issue, there is rarely any systematic control of this - the case study simply illustrates a single successful application of this treatment. The selection of the case to be described is usually based on a dramatic demonstration of a new or unusual form of treatment, and the impression created is often very persuasive. It is very important then to distinguish this compelling impression from the very limited actual evidence represented by a case study about causal effects.