OBJECTIVES: The objective of the ‘piggy-back’ trial is to examine the cost-effectiveness of infliximab compared to etanercept in patients with moderate to severe plaque psoriasis or psoriasis vulgaris. Before starting the cost-effectiveness study it is usefull to know what to measure and where to invest or aim for. The objective of this paper is to estimate the expected value of perfect information (EVPI), with an underlying goal of estimating the partial EVPI’s (EVPPI’s) to make a prediction about the value of obtaining further information, for all parameters and a partial set of parameters. METHODS: Analysis was conducted using a Markov model for patients with moderate to severe plaque psoriasis. For estimating partial EVPI’s (EVPPI’s) a Monte Carlo simulation (MCS) method was used. Transition probabilities were calculated, based on published evidence, expert opinion, and demographic data. Outcomes expected were total societal costs, expected QALY’s and clinical effectiveness. The analysis was performed from a partial societal perspective of The Netherlands. The outcome of partial EVPI was split into costs, utilities, success rates and dropout rates. RESULTS: The cost-effectiveness acceptability curve (CEAC) indicates a high decision uncertainty. The CEAC and EVPI also show infliximab needs a high willingness to pay. According to the EVPPI analysis the most uncertainty is seen in utilities (25,239 million) followed by costs (€4,216 million). Success rates and dropout rates also show a high EVPPI but much lower (around 204 and 385 million). CONCLUSIONS: When looking at the EVPPI’s there is clearly much interest in investing in research to the utilities and the cost of treatment and little interest investing in success rates and progress rates. Because indirect costs, like costs of travel and productivity loss, are excluded and differ between the two therapies, there can be potential gain by further research to the costs.