hta_ knee intro.qxp - Ministero della Salute

Appendix 3
Sampling methodology
The sample consists of a proportional stratified sample with random extraction of the units.
The sample was:
- stratified as the population is subdivided into subpopulations called strata;
- proportional as the allocation of sample units at different strata retain the same proportion
with which they are present in the population;
- random as the sample units are drawn randomly from a list of units.
The sample was calculate as:
where
n = sample size;
ε = standard error (set at 0,025);
N = Population (centres that had undertaken at least one primary total knee replacement
or revision);
P = probability of primary total knee replacement is done;
Q = probability of revision total knee replacement is done;
P and Q (calculate using Hospital discharge records - SDO) were reported in Table A2.1.
After this stage sampling was made taking into account the proportional distribution of the population
compared with strata (by Italian region) and substrate (by volume of activity) as follows:
- population strata of 21 Italian Regions and Autonomous Provinces (P.A.);
- determining the sample by strata substrata (three substrate for each strata);
- random extraction within substrata.
Sampling units was extracted using the following rate to each strata:
preserving, for each stratum, the proportions of structures for volumes of activity.
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