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153<br />

again from September to November. Of prime interest here are <strong>the</strong> first three years <strong>of</strong><br />

survey observations to 1986.<br />

The ALS data finally used for analysis has a reasonable rate <strong>of</strong> sample reduction. Each<br />

year, fewer observations in ALS have survey outcomes available for analysis. The sample<br />

reduction due to attrition is shown in Table 5.1. The initial non-response rate for <strong>the</strong> first<br />

observations in 1984 is 20 per cent. Loss <strong>of</strong> sample observations between <strong>the</strong> first survey<br />

in1984 and <strong>the</strong> second survey in 1985, is 21 per cent. The attrition rate falls to 10 per cent<br />

between 1985 and 1986. By <strong>the</strong> third survey in 1986, only 57% <strong>of</strong> <strong>the</strong> original sample<br />

remains. Non-response was addressed in <strong>the</strong> survey design with heavy follow-up 96 . The<br />

final sample <strong>of</strong> 1283 cases used for analysis 97 arises due to analytical selection, where<br />

404 cases were discarded, amounting to 24 per cent <strong>of</strong> those who respond to all surveys<br />

to 1986. These different sources <strong>of</strong> sample reduction are now examined.<br />

Table 5.1 Sample Reduction for ALS List sample<br />

Number <strong>of</strong> cases<br />

% cases lost<br />

each stage<br />

Original sample 2998<br />

1984 survey interviews 2403 20 80<br />

1985 survey interviews 1910 21 64<br />

1986 survey interviews 1711 10 57<br />

1987 survey interviews 1518 11 51<br />

% cases<br />

remaining <strong>of</strong><br />

original<br />

sample<br />

A large part <strong>of</strong> <strong>the</strong> sample reduction in <strong>the</strong> analytical selection is due to missing data<br />

problems. Of those responding to <strong>the</strong> 1986 survey, that is, <strong>of</strong> those that do not attrit,<br />

fur<strong>the</strong>r sample reduction occurs for several reasons. As previously described from those<br />

interviewed in 1984, <strong>the</strong> final sample excludes those who were over 25 years at <strong>the</strong><br />

96 Interviewers were required to make 7 visits in person to <strong>the</strong> initial, 2 nd or 3 rd address <strong>of</strong> a mover, and<br />

were instructed to use a varying time <strong>of</strong> contact pattern, and supervisors would follow-up in attempts to<br />

convert refusals. Interpreters were used where necessary, and an introductory approach letter from <strong>the</strong><br />

Minister and <strong>the</strong> survey firm was sent in advance <strong>of</strong> surveying. All <strong>of</strong> <strong>the</strong>se took place before non-contact<br />

was declared.<br />

97 The sample is formed to match that <strong>of</strong> Richardson (1998) to facilitate replication and subsequent<br />

comparison <strong>of</strong> results.

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