Description

Contingency tables and chisquared tests of association for survey data.

Usage

Arguments

Details

The svytable function computes a weighted crosstabulation. This is especially useful for producing graphics. It is sometimes easier to use svytotal or svymean, which also produce standard errors, design effects, etc.

The frequencies in the table can be normalised to some convenient total such as 100 or 1.0 by specifying the Ntotal argument. If the formula has a left-hand side the mean or sum of this variable rather than the frequency is tabulated.

The Ntotal argument can be either a single number or a data frame whose first column gives the (first-stage) sampling strata and second column the population size in each stratum. In this second case the svytable command performs ‘post-stratification’: tabulating and scaling to the population within strata and then adding up the strata.

As with other xtabs objects, the output of svytable can be processed by ftable for more attractive display. The summary method for svytable objects calls svychisq for a test of independence.

svychisq computes first and second-order Rao-Scott corrections to the Pearson chisquared test, and two Wald-type tests.

The default (statistic="F") is the Rao-Scott second-order correction. The p-values are computed with a Satterthwaite approximation to the distribution and with denominator degrees of freedom as recommended by Thomas and Rao (1990). The alternative statistic="Chisq" adjusts the Pearson chisquared statistic by a design effect estimate and then compares it to the chisquared distribution it would have under simple random sampling.

The statistic="Wald" test is that proposed by Koch et al (1975) and used by the SUDAAN software package. It is a Wald test based on the differences between the observed cells counts and those expected under independence. The adjustment given by statistic="adjWald" reduces the statistic when the number of PSUs is small compared to the number of degrees of freedom of the test. Thomas and Rao (1990) compare these tests and find the adjustment benefical.

statistic="lincom" replaces the numerator of the Rao-Scott F with the exact asymptotic distribution, which is a linear combination of chi-squared variables (see pchisqsum, and statistic="saddlepoint" uses a saddlepoint approximation to this distribution. The CompQuadForm package is needed for statistic="lincom" but not for statistic="saddlepoint". The saddlepoint approximation is especially useful when the p-value is very small (as in large-scale multiple testing problems).

For designs using replicate weights the code is essentially the same as for designs with sampling structure, since the necessary variance computations are done by the appropriate methods of svytotal and svymean. The exception is that the degrees of freedom is computed as one less than the rank of the matrix of replicate weights (by degf).

At the moment, svychisq works only for 2-dimensional tables.

Value

The table commands return an xtabs object, svychisq returns a htest object.

Note

Rao and Scott (1984) leave open one computational issue. In computing ‘generalised design effects’ for these tests, should the variance under simple random sampling be estimated using the observed proportions or the the predicted proportions under the null hypothesis? svychisq uses the observed proportions, following simulations by Sribney (1998), and the choices made in Stata

References

Davies RB (1973). "Numerical inversion of a characteristic function" Biometrika 60:415-7

P. Duchesne, P. Lafaye de Micheaux (2010) "Computing the distribution of quadratic forms: Further comparisons between the Liu-Tang-Zhang approximation and exact methods", Computational Statistics and Data Analysis, Volume 54, 858-862

Koch, GG, Freeman, DH, Freeman, JL (1975) "Strategies in the multivariate analysis of data from complex surveys" International Statistical Review 43: 59-78

Rao, JNK, Scott, AJ (1984) "On Chi-squared Tests For Multiway Contigency Tables with Proportions Estimated From Survey Data" Annals of Statistics 12:46-60.

Sribney WM (1998) "Two-way contingency tables for survey or clustered data" Stata Technical Bulletin 45:33-49.

Thomas, DR, Rao, JNK (1990) "Small-sample comparison of level and power for simple goodness-of-fit statistics under cluster sampling" JASA 82:630-636

See Also

svytotal and svymean report totals and proportions by category for factor variables.

See svyby and ftable.svystat to construct more complex tables of summary statistics.

See svyloglin for loglinear models.

See regTermTest for Rao-Scott tests in regression models.

See https://notstatschat.rbind.io/2019/06/08/design-degrees-of-freedom-brief-note/ for an explanation of the design degrees of freedom with replicate weights.

Examples