Since my post on Sunday a number of new polls have come out. This post shows the results of the meta-analysis using the six polls whose results have come out since Sunday.
Meta-analysis of last week’s Scottish independence polls
As most readers will know, this Thursday (18th September 2014), residents of Scotland will vote in a referendum to decide whether to become independent of the UK. While the No campaign had previously maintained a reasonably healthy lead against Yes, in recent weeks the race has tightened considerably, on the basis of polls of voting intentions. In particular, two polls have now shown larger proportions saying they will vote Yes compared to the proportions voting No. With a flurry of polls conducted in the last week, each with slightly different results, I decided to perform a simple meta-analysis of the poll results, to estimate the current state of play, based on the available evidence.
Checking functional form in logistic regression using loess plots
When we include a continuous variable as a covariate in a regression model, it’s important that we include it using the correct (or something approximately correct) functional form. For example, with a continuous outcome Y and continuous covariate X, it may be the case that the expected value of Y is a linear function of X and X^2, rather than a linear function of X. For linear regression there are a number of ways of assessing what the appropriate functional form is for a covariate. A simple but often effective approach is simply to look at a scatter plot of Y against X, to visually assess the shape of the association.