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Using an Inappropriate Method of Analysis

"... all models are limited by the validity of the assumptions on which they ride."
David Collier, Jasjeet S. Sekhon, and Philip B. Stark, Preface (p. xi) to Freedman David A., Statistical Models and Causal Inference: A Dialogue with the Social Sciences.

"Assumptions behind models are rarely articulated, let alone defended. The problem is exacerbated because journals tend to favor a mild degree of novelty in statistical procedures. Modeling, the search for significance, the preference for novelty, and the lack of interest in assumptions -- these norms are likely to generate a flood of nonreproducible results."
David Freedman, Chance 2008, v. 21 No 1, p. 60

Each frequentist1 inference technique (hypothesis test or confidence interval) involves model assumptions. Different techniques have different model assumptions. The validity of the technique depends (to varying extents) on whether or not the model assumptions are true for the context of the data being analyzed.

Many techniques are robust to departures from at least some model assumptions. This means that if the particular assumption is not too far from true, then the technique is still approximately valid.2 Thus, when using a statistical technique, it is important to ask:
Neglecting these questions is a common mistake in using statistics. Sometimes researchers check only some of the assumptions, perhaps missing some of the most important ones.

Unfortunately, the model assumptions vary from technique to technique, so there are few if any general rules. One general rule of thumb, however is:

Techniques are least likely to be robust to departures from assumptions of independence.3, 4

Note: Assumptions of independence are often phrased in terms of "random sample" or "random assignment", so these are very important.

"The independence assumption is fragile. ... Even modest violations of independence can introduce substantial biases into conventional procedures."
David A. Freedman, Statistical Models and Causal Inference: A Dialogue with the Social Sciences, p. 31

" The independence assumption ... is a dangerous assumption in practice!"
Bradley Efron, Large Scale Inference, p. 26

How do I know whether or not model assumptions are satisfied?

Unfortunately, there are no one-size-fits-all methods, but here are some rough guidelines that can help sometimes:

1. When selecting samples or dividing into treatment groups, be very careful in randomizing according to the requirements of the method of analysis to be used.

See What is a Random Sample? and further links from that page for more detail.
See also:
 Biased Sampling and Extrapolation for some examples of how the sampling method may result in a problematical sample.
Analyzing Data Without Regard to How They Were Collected

2. Sometimes (not too often) model assumptions can be justified plausibly by well-established5 facts, mathematical theorems, or theory that is well-supported by sound empirical evidence.


3. Sometimes a rough idea of whether or not model assumptions might fit can be obtained by either plotting the data or plotting residuals obtained from a tentative use of the model.

Note: Unfortunately, these methods are typically better at telling you when the model assumption does not fit than when it does.

Examples, Guidelines, and Cautions

Specific Situations Where Mistakes Involving Model Assumptions Are Often Made

Using a two-sample test comparing means when cases are paired (also includes discussion of repeated measures)

Comparisons of treatments applied to people, animals, etc  (Intent to Treat; Comparisons involving Drop-outs)

Fixed vs Random Factors

Analyzing Data without Regard to How the Data Were Collected 

Dividing a Continuous Variable into Categories ("Chopped Data")


Mistakes in Regression

Dealing with Missing Data
For More Discussion of Inappropriate Methods of Analysis

1. Bayesian statistical techniques also involve assumptions; this web site focuses mostly on frequentist techniques.
2. The Rice Virtual Lab in Statistics' Robustness Simulation can be used to demonstrate the effect of some violations of model assumptions on the two-sample t-test. 
3. However, there is some robustness to some types of departures from independence. One is that, for large enough populations, sampling without replacement is good enough, even though "independent" technically means sampling with replacement; see More Precise Definition of Simple Random Sample
4. For more discussion of the independence assumption and possible effects of violations of it, see the Freedman (2010) reference above, especially chapters 1 - 3 and 19.
5. Here, "well established" means well established by sound empirical evidence and/or sound mathematical reasoning. This is not the same as "well-accepted," since sometimes things may be well-accepted without sound evidence or reasoning.

Last updated August 28, 2012