Interpretation of DQASS reports

Crude and standardised SPR

Confidence intervals • For SPR • For MoM values • The importance of sample

size • Practical session • The need to look at

CUSUM charts

Flags for markers

CONFIDENCE

INTERVALS

Confidence for screen positive rates

Confidence for MoM values

MoM values should be centred on 1.0 when viewed together, by gestation, by weight, by smoking status, by ethnicity … Small deviations from this target can have important implications for screening

Confidence Intervals

• Confidence intervals are used extensively in DQASS reports for screen positive rates, median MoM values and other unknown parameters

• A confidence is an interval that contains the truth with a specific level of confidence (usually 95%)

• If a confidence interval for a median MoM value includes 1, there is little evidence to conclude that the (true) median MoM differs from 1

Confidence Intervals

• A confidence is an interval that contains the truth at a specific level of confidence (usually 95%)

TRUTH

×

The importance of sample size

Med

ian

Mo

M

0.80

0.85

0.90

0.95

1.00

1.05

1.10

1.15

1.20 n = 50

n = 100

n = 200n = 300

n = 400 n = 500

n = 50

n = 100

n = 200n = 300

n = 400 n = 500

Larger samples More precise estimates Shorter confidence intervals

Practical Session M

ed

ian

Mo

M

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5 n = 78 Can substantive biases be ruled out? Is there evidence of substantial bias? What is the direction and magnitude of the bias? How much uncertainty is there? It is always important to check the CUSUM chart to Examine consistency over time.

Practical Session M

ed

ian

Mo

M

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5 n = 78

There is evidence of a substantive positive bias in median MoM values. The estimated median MoM is around 1.25. Due to the limited amount of data, there is considerable uncertainty. The 95% confidence interval rages from just above 1.1 to over 1.4.

As always, CUSUM charts should be inspected for changes over time.

Why is it important to look at CUSUM charts

-25% first half +20% second half

Med

ian

Mo

M

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

n = 1000

Why is it important to look at CUSUM charts

20% bias for first 75%, no bias for last 25%.

Med

ian

Mo

M

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

n = 1000

Flags for marker median MoM – 10% rule

99.13 99.12 99.13 99.14

Flags for marker median MoM

– 5% rule

99.12 99.13 99.14

Crude and standardised screen

positive rates

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Monitoring screen positive rates

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225/7736 = 0.029 (2.9%)

Standardising screen positive rates

• Crude screen positive rate

Proportion of population screened

positive

• Standardised screen positive rate

Proportion of standard population

screened positive

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Combined test

T21 1.8 -2.5% T18/13 0.1 - 0.2% Both 1.8 - 2.5%

Quadruple test

T21 2.5-3.5%

Programme standards for standardised SPR

Crude screen positive rates

• The crude screen positive rate is important because it relates directly to the particular population

• In general, for a given test, the crude screen positive rate depends on the distribution of maternal ages

• Comparisons of crude screen positive rates for different populations are difficult to interpret because of this dependency on maternal age distribution

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Standardised screen positive rates

• The standardised screen positive rate is the proportion of women that would screen positive for a specified reference maternal age distribution (e.g. the distribution of maternal ages over the three year period 2000-2002)

• Standardised screen positive rates obtained from different populations are comparable with each other

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