Sample size considerations: It is a well-known difficulty for exploratory prognostic factor research studies like this, to formalize how many participants (i.e. with events) to include. In order to consider an adequate number of outcome events, the investigators apply "the rule of thumb" that dictates that 10 outcome events are needed for each independent variable (possible predictors); the investigators plan to enrol 320 patients in total, and anticipate that 50% of these will experience a clinical response during the 14-16 week period after therapy with TNF inhibitors is initiated. With this in mind: Anticipating that at least 160 will achieve clinical responses (among the 320 patients), this study will have a reasonable power to explore the impact of as many as 16 independent (predictor) variables (including condition and clinical centre).