Add information about the different degrees of freedom default to the `srvyr` vs `survey` vignette

[florisvdh]

It appears that the 95% confidence limits from survey_mean() are slightly different from those obtained with survey::svymean() - is this a bug?

suppressPackageStartupMessages({
    library(survey)
    library(srvyr)
})
data(api)

srs_design <- apisrs %>% svydesign(id = ~1, fpc = ~fpc, data = .)
api_mean <- svymean(~enroll, srs_design)
(svy_ci <- confint(api_mean))
#>          2.5 %  97.5 %
#> enroll 530.969 638.251

srs_design_srvyr <- apisrs %>% as_survey_design(ids = 1, fpc = fpc)
(srvyr_ci <- 
    srs_design_srvyr %>% 
    summarise(enroll = survey_mean(enroll, vartype = "ci")))
#>   enroll enroll_low enroll_upp
#> 1 584.61   530.6408   638.5792

svy_ci[1,1] == srvyr_ci$enroll_low
#> [1] FALSE
svy_ci[1,2] == srvyr_ci$enroll_upp
#> [1] FALSE

svy_ci[1,1] == coef(api_mean) - qnorm(0.975) * SE(api_mean)
#>        enroll
#> enroll   TRUE
svy_ci[1,2] == coef(api_mean) + qnorm(0.975) * SE(api_mean)
#>        enroll
#> enroll   TRUE

^{Created on 2021-05-07 by the reprex package (v2.0.0)}

Session info

sessioninfo::session_info()
#> ─ Session info ───────────────────────────────────────────────────────────────
#>  setting  value                       
#>  version  R version 4.0.5 (2021-03-31)
#>  os       Linux Mint 20               
#>  system   x86_64, linux-gnu           
#>  ui       X11                         
#>  language nl_BE:nl                    
#>  collate  nl_BE.UTF-8                 
#>  ctype    nl_BE.UTF-8                 
#>  tz       Europe/Brussels             
#>  date     2021-05-07                  
#> 
#> ─ Packages ───────────────────────────────────────────────────────────────────
#>  package     * version date       lib source        
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#> [3] /usr/lib/R/site-library
#> [4] /usr/lib/R/library

[gergness]

Not a bug (though a little confusing, I admit). srvyr tries to be helpful by setting the df on confint for you.

If you prefer the results you got using the survey package, you can do this in srvyr:

(srvyr_ci_2 <- 
        srs_design_srvyr %>% 
        summarise(enroll = survey_mean(enroll, vartype = "ci", df = Inf)))
#>   enroll enroll_low enroll_upp
#> 1 584.61    530.969    638.251

Or to get the srvyr results in survey:

api_mean_2 <- svymean(~enroll, srs_design)
(svy_ci_2 <- confint(api_mean_2, df = degf(srs_design)))
#>           2.5 %   97.5 %
#> enroll 530.6408 638.5792

[florisvdh]

Thank you for explaining @gergness. And indeed the df argument is present in the documentation of survey_mean() :+1:.

I think it would be good to have this aspect added in the srvyr-vs-survey vignette. The difference can be seen under heading Summary statistics, but it's not discussed.

> svy_ci_2[1,1] == srvyr_ci$enroll_low
[1] TRUE
> svy_ci_2[1,2] == srvyr_ci$enroll_upp
[1] TRUE
> 
> srvyr_ci$enroll_low == coef(api_mean) - qt(0.975, df = degf(srs_design)) * SE(api_mean)
       enroll
enroll   TRUE
> srvyr_ci$enroll_upp == coef(api_mean) + qt(0.975, df = degf(srs_design)) * SE(api_mean)
       enroll
enroll   TRUE
> 
> qt(0.975, df=Inf) == qnorm(0.975)
[1] TRUE

[gergness]

yeah, makes sense