13.07.2015 Views

A Simple Poverty Scorecard for the Philippines

A Simple Poverty Scorecard for the Philippines

A Simple Poverty Scorecard for the Philippines

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

interval of +/–0.44 percentage points (Figure 9). 22 Plugging <strong>the</strong>se into <strong>the</strong> directmeasurementsample-size <strong>for</strong>mula (1) above gives not n = 16,384 but ra<strong>the</strong>rn=⎛ 1.64 ⎞⎜ ⎟⎝ 0.0044 ⎠2⋅ 0.313 ⋅ ( 1 − 0.313)= 29,874. The ratio of <strong>the</strong> sample size <strong>for</strong> scoring(derived empirically) to <strong>the</strong> sample size <strong>for</strong> direct measurement (derived from <strong>the</strong>ory) is16,384 ÷ 29,874 = 0.55.Applying <strong>the</strong> same method to n = 8,192 (confidence interval of +/–0.6221.64percentage points) gives n =⎛ ⎞⎜ ⎟ ⋅ 0.313 ⋅ ( 1 − 0.313)= 15,046. This time, <strong>the</strong>⎝ 0.0062 ⎠ratio of <strong>the</strong> sample size using scoring to <strong>the</strong> sample size using direct measurement is8,192 ÷ 15,046 = 0.54. This ratio <strong>for</strong> n = 8,192 is close to that <strong>for</strong> n = 16,384. Indeed,applying this same procedure <strong>for</strong> all n ≥ 256 in Figure 9 gives ratios that average to0.55.This approach can be used to define a sample-size <strong>for</strong>mula <strong>for</strong> <strong>the</strong> povertyscorecard applied to <strong>the</strong> population in <strong>the</strong> validation sample:n2z= α ⋅⎛ ⎞⎜ ⎟ ⋅ pˆ⋅ ( 1 − pˆ), (2)⎝ c ⎠where α = 0.55 and z, c, and pˆ are defined as in (1) above. It is this α that appears inFigure 8 as “α <strong>for</strong> sample size”.To illustrate <strong>the</strong> use of (2), suppose c = 0.0359 (confidence interval of +/–3.59percentage points) and z = 1.64 (90-percent confidence). Then (2) gives22Due to rounding, Figure 9 displays 0.4, not 0.44.31

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!