11 By allowing individuals to be followed over time, longitudinal or panel data constitute the appropriate instrument for this type of analysis and for the measurement of the degree of persistence of unemployed on specific groups or individuals, Following Mincer (1982), let us define P as the incidence of unemployment over a period of time (for example, over one, two or three years). If unemployment is repeatedly experienced by the same individuals, the incidence remains the same, whether or not measured over one, two or three year period , P, = P2 = P3. If on the contrary, the group is homogeneous and there is perfect turnover with a probability Pi = P for each individual, over any single time period, the proportion of the labor force who experiences unemployment spells will grow over time from P to an upper limit: when the observation period reaches n-time periods. The actual comparison between the observed n- year incidence P. and the value of this upper limit P. max allows for an estimatior of the degree of persistence (or turnover) of the phenomenon.

The panel data from the Mexican Urban Employment survey (ENEU) allow to follow the same individuals from the 3rd quarter of 1990 to the 3rd quarter of 1991, that is over one full year, with a snapshot every quarter. The question on unemployment unfortunately only refers to the week preceding the survey rather than to the whole three-month period. As a result, the data provide information over the five successive snapshots but there is need to correct the information gap between each of them. For this reason, we proceed in several steps: (i) We estimate the incidence of unemployment from each of the five successive quarterly surveys. An average incidence P, is calculated. It refers to a one-week observation period.

(ii) We calculate the incidence observed over the five consecative surveys P,-. This measures the proportion of the labor force observed hit by unemployment at least once ( a person found unemployed in several snapshots is only counted once).

(iii) We attempt to estimate the number of workers who both enter and exit unemployment between two successive surveys and therefore, escape our accounting process done at threemonth intervals. The number of these workers is inferred from the analysis of duration of unemployment spells presented in the previous section. From this analysis, information is drawn on the size of each cohort entering unemployment over one month, and the proportion exiting before three months. Estimating the number of these short-term unemployed workers who escape our accounting between our different observation points allows to draw some inference on the incidence of unemployment over a three-month period and thus, correct estimate (i).

(iv) Based on (iii), che incidence corresponding to a full-year period is calculated: Py. corr. Lower and upper bound estimates are calculated . Lower-bound estimates correspond to the case in which the short-term unemployed (with duration inferior to three months) who cannot be captured from a survey to the next, are repeatedly hit during, each quarter (perfect persistence or heterogeneity). The upper-bound estimate corresponds to the opposite case (perfect turnover or homogeneity): a "new" group of short-term unemployed enters each quarter.