flynn002

Intuitive explanations of statistical concepts for novices #4 The p-value is widely used but widely misunderstood. I'll demonstrate this in the context of intervention studies. The key question is how confident can we be that an apparently beneficial effect of treatment reflects a change due to the intervention, rather than arising just through the play of chance. The p-value gives one way of deciding that. There are other approaches, including those based on Bayesian statistics, which are preferred by many statisticians. But I will focus here on the traditional null hypothesis significance testing (NHST) approach, which dominates statistical reporting in many areas of science, and which uses p-values. As illustrated in my previous blogpost , where our measures include random noise, the distorting effects of chance mean that we can never be certain whether or not a particular pattern of data reflects a real difference between groups. However, we can compute the probability that the...

Intuitive explanations of statistical concepts for novices #3 I'll be focusing here on the kinds of stats needed if you conduct an intervention study. Suppose we measured the number of words children could define on a 20-word vocabulary task. Words were selected so that at the start of training, none of the children knew any of them. At the end of 3 months of training, every child in the vocabulary training group (B) knew four words, whereas those in a control group (A) knew three words. If we had 10 children per group, the plot of final scores would look like Figure 1 panel 1. Figure 1. Fictional data to demonstrate concept of random error (noise) In practice, intervention data never look like this. There is always unexplained variation in intervention outcomes, and real results look more like panel 2 or panel 3. That is, in each group, some children learn more than average and some less than average. Such fluctuations can reflect numerous sources of uncontrolled variation: for in...