Description: The use of free school meal data is prevalent in official estimates of educational disadvantage as it is in educational research reports in Britain. However, while there has been some concern expressed about the measure, there has, to our knowledge, been no systematic test of its appropriateness. The appropriateness of a measure will always depend upon the questions being asked so we are not suggesting the measure will always and everywhere be inappropriate. But we do think there is an issue worthy of debate relating to the kinds of questions that research seeks to address. The paper will begin by outlining the kinds of educational research that uses this measure, with some discussion of the conditions under which it may be considered appropriate and those where its relevance needs to be tested. In particular, judgements about school performance that use this measure may be criticised on two grounds. The first and more widely acknowledged, concerns the approximate nature of FSM eligibility as a measure of social deprivation. The second concerns the possibility that the population of those on FSMs is highly unstable and that any calculation or judgement is likely, therefore, to be an underestimate of the real disadvantage that a school confronts. It is with this latter possibility that this paper is concerned. For example, if the pool of students who take up FSMs over a given period is larger than those counted at a specific time then the proportion of disadvantaged in a school will be higher than acknowledged. In order to understand why this measure may problematic some background in terms of the nature of the flexible labour market in Britain and the distribution of welfare benefits related to it is necessary since both impact on the nature of child poverty of which FSM is assumed to be a reliable indicator. The paper estimates the proportion of children who have taken up FSMs over time using data from Hampshire schools and models the revised estimates in order to assess their impact on school outcomes. Methodology: Quantitative Conclusions: That there is considerable variability over time in those who take up FSMs with the consequence that the proportion of disadvantaged students in a school is likely to be underestimated.