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Table 1 Prevalences of selected behaviours, 2013–2015: mean, 95% confidence interval (95% CI) and design effect (DEFF)

From: Repeat participation in annual cross-sectional surveys of drug users and its implications for analysis

Self-reported behaviour

Meana

95% CI

DEFF1b

DEFF2c

Used heroin, ever (n = 2565)

.88

(.86, .89)

1.07

1.07

Used heroin, last 6 months (n = 2565)

.59

(.57, .61)

1.06

1.05

Used ice, ever (n = 2562)

.81

(.79, .82)

1.06

1.06

Used ice, last 6 months (n = 2564)

.61

(.59, .63)

1.04

1.04

Injected with a needle used by someone else, last month (n = 2489)

.07

(.06, .08)

1.02

1.02

No. of heroin overdoses, ever (n = 2081)

2.3

(1.8, 2.7)

1.01

1.01

No. of injections, last month (n = 2426)

38.3

(36.1, 40.5)

1.03

1.03

  1. a Indicates proportions for binary measures and average counts for interval measures
  2. b DEFF1 = ratio of the variance of the complex estimator to that from estimation assuming SRS (e.g. DEFF of 1.06 indicates the complex sample is 6% less efficient in estimating prevalence than an SRS)
  3. c DEFF2 = ratio of the variance of the jackknife estimator accounting for participant clustering in response to that from jackknife estimation without accounting for clustering
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