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Descriptives statistics for Table 1

Max Gordon

2023-08-25

The purpose of the first table in a medical paper is most often to describe your population. In an RCT the table frequently compares the baseline characteristics between the randomized groups, while an observational study will often compare exposed with unexposed. In this vignette I will show how I use the functions to quickly generate a descriptive table.

We will use the mtcars dataset and compare the groups with automatic transmission to those without. The units and labels are built upon the logic in the Hmisc package that allow us to specify attributes on columns. Note that this labeling is not needed, it just makes stuff nicer.

library(Gmisc)
data("mtcars")
mtcars <- mtcars %>% 
  mutate(am = factor(am, levels = 0:1, labels = c("Automatic", "Manual")),
         gear = factor(gear),
         # Make up some data for making it slightly more interesting
         col = factor(sample(c("red", "black", "silver"),
                             size = NROW(mtcars), 
                             replace = TRUE))) %>% 
  set_column_labels(mpg = "Gas",
                    wt = "Weight",
                    am = "Transmission",
                    gear = "Gears",
                    col = "Car color") %>% 
  set_column_units(mpg = "Miles/(US) gallon",
                   wt = "10<sup>3</sup> lbs")

The basics of getDescriptionStatsBy

The function getDescriptionStatsBy is a simple way to do basic descriptive statistics. Mandatory named column is by:

mtcars %>% 
  getDescriptionStatsBy(mpg, 
                        wt,
                        am,
                        gear,
                        col,
                        by = am)
Automatic Manual
Gas 17.1 (±3.8) 24.4 (±6.2)
Weight 3.8 (±0.8) 2.4 (±0.6)
Transmission 19 (100.0%) 0 (0.0%)
Gears
  3 15 (78.9%) 0 (0.0%)
  4 4 (21.1%) 8 (61.5%)
  5 0 (0.0%) 5 (38.5%)
Car color
  black 8 (42.1%) 3 (23.1%)
  red 5 (26.3%) 4 (30.8%)
  silver 6 (31.6%) 6 (46.2%)

If we prefer median we can simply specify the statistic used with continuous variables:

mtcars %>% 
  getDescriptionStatsBy(mpg, 
                        wt,
                        am,
                        gear,
                        col,
                        by = am,
                        continuous_fn = describeMedian)
Automatic Manual
Gas 17.3 (14.9 - 19.2) 22.8 (21.0 - 30.4)
Weight 3.5 (3.4 - 3.8) 2.3 (1.9 - 2.8)
Transmission 19 (100.0%) 0 (0.0%)
Gears
  3 15 (78.9%) 0 (0.0%)
  4 4 (21.1%) 8 (61.5%)
  5 0 (0.0%) 5 (38.5%)
Car color
  black 8 (42.1%) 3 (23.1%)
  red 5 (26.3%) 4 (30.8%)
  silver 6 (31.6%) 6 (46.2%)

Integration with htmlTable

Key to having a good descriptive statistics is to be able to output it into a table. I usually rely on htmlTabl for all my table requirements as it has a nice set of advanced options that allow us to get publication ready tables that can simply be copy-pasted into our paper. Note that we here use html code † that we then explain in the footer. If we specify a name to the parameters like this we override the labels previously set.

mtcars %>% 
  getDescriptionStatsBy(mpg, 
                        `Weight&dagger;` = wt,
                        am,
                        gear,
                        col,
                        by = am) %>% 
  htmlTable(caption  = "Basic descriptive statistics from the mtcars dataset",
            tfoot = "&dagger; The weight is in 10<sup>3</sup> kg")
Basic descriptive statistics from the mtcars dataset
Automatic Manual
Gas 17.1 (±3.8) 24.4 (±6.2)
Weight† 3.8 (±0.8) 2.4 (±0.6)
Transmission 19 (100.0%) 0 (0.0%)
Gears
  3 15 (78.9%) 0 (0.0%)
  4 4 (21.1%) 8 (61.5%)
  5 0 (0.0%) 5 (38.5%)
Car color
  black 8 (42.1%) 3 (23.1%)
  red 5 (26.3%) 4 (30.8%)
  silver 6 (31.6%) 6 (46.2%)
† The weight is in 103 kg

Extra everything

There is a large set of options for getDescriptionStatsBy, here is an example with some of them an some extra styling.

mtcars %>% 
  getDescriptionStatsBy(mpg, 
                        `Weight&dagger;` = wt,
                        am,
                        gear,
                        col,
                        by = am,
                        digits = 0,
                        add_total_col = TRUE,
                        use_units = "name") %>% 
  addHtmlTableStyle(pos.caption = "bottom") %>% 
  htmlTable(caption  = "Basic descriptive statistics from the mtcars dataset",
            tfoot = "&dagger; The weight is in 10<sup>3</sup> kg")
Total Automatic Manual
Gas (Miles/(US) gallon) 20 (±6) 17 (±4) 24 (±6)
Weight† (103 lbs) 3 (±1) 4 (±1) 2 (±1)
Transmission 19 (59%) 19 (100%) 0 (0%)
Gears
  3 15 (47%) 15 (79%) 0 (0%)
  4 12 (38%) 4 (21%) 8 (62%)
  5 5 (16%) 0 (0%) 5 (38%)
Car color
  black 11 (34%) 8 (42%) 3 (23%)
  red 9 (28%) 5 (26%) 4 (31%)
  silver 12 (38%) 6 (32%) 6 (46%)
Basic descriptive statistics from the mtcars dataset
† The weight is in 103 kg

P-values

Event though p-values are discouraged in the Table 1, they are not uncommon. I have therefore added basic statistics consisting that defaults to Fisher’s exact test for proportions and Wilcoxon rank sum test for continuous values.

mtcars %>% 
  getDescriptionStatsBy(mpg, 
                        wt,
                        am,
                        gear,
                        col,
                        by = am,
                        continuous_fn = describeMedian,
                        digits = 0,
                        header_count = TRUE,
                        statistics = TRUE) %>% 
  htmlTable(caption  = "Basic descriptive statistics from the mtcars dataset")
Basic descriptive statistics from the mtcars dataset
Automatic
No. 19 		Manual
No. 13 		P-value
Gas 17 (15 - 19) 23 (21 - 30) 0.002
Weight 4 (3 - 4) 2 (2 - 3) < 0.0001
Transmission 19 (100%) 0 (0%) < 0.0001
Gears < 0.0001
  3 15 (79%) 0 (0%)
  4 4 (21%) 8 (62%)
  5 0 (0%) 5 (38%)
Car color 0.60
  black 8 (42%) 3 (23%)
  red 5 (26%) 4 (31%)
  silver 6 (32%) 6 (46%)

Custom p-values

By popular demand I’ve expanded with the option of having custom p-values. All you need to do is to provide a function that takes two values and exports a single p-value. There are several prepared functions that you can use or use as a template for your own p-value function. They all start with getPval.., e.g. getPvalKruskal. You can either provide a single function or you can set the defaults depending on the variable type:

mtcars %>% 
  getDescriptionStatsBy(mpg, 
                        wt,
                        am,
                        gear,
                        col,
                        by = am,
                        continuous_fn = describeMedian,
                        digits = 0,
                        header_count = TRUE,
                        statistics = list(continuous = getPvalChiSq, 
                                          factor = getPvalChiSq, 
                                          proportion = getPvalFisher)) %>% 
  addHtmlTableStyle(pos.caption = "bottom") %>% 
  htmlTable(caption  = "P-values generated from a custom set of values")
Automatic
No. 19 		Manual
No. 13 		P-value
Gas 17 (15 - 19) 23 (21 - 30) 0.27
Weight 4 (3 - 4) 2 (2 - 3) 0.37
Transmission 19 (100%) 0 (0%) < 0.0001
Gears < 0.0001
  3 15 (79%) 0 (0%)
  4 4 (21%) 8 (62%)
  5 0 (0%) 5 (38%)
Car color 0.52
  black 8 (42%) 3 (23%)
  red 5 (26%) 4 (31%)
  silver 6 (32%) 6 (46%)
P-values generated from a custom set of values

Using mergeDesc

Prior to Gmisc v3.0 mergeDesc was the best way to quickly assemble a “Table 1”:

getTable1Stats <- function(x, digits = 0, ...){
  getDescriptionStatsBy(x = x, 
                        by = mtcars$am,
                        digits = digits,
                        continuous_fn = describeMedian,
                        header_count = TRUE,
                        ...)
  
}

t1 <- list()
t1[["Gas"]] <-
  getTable1Stats(mtcars$mpg)
  
t1[["Weight&dagger;"]] <-
  getTable1Stats(mtcars$wt)

t1[["Color"]] <- 
  getTable1Stats(mtcars$col)

# If we want to use the labels set in the beginning
# we add an element without a name
t1 <- c(t1,
        list(getTable1Stats(mtcars$gear)))

mergeDesc(t1,
          htmlTable_args = list(caption  = "Basic descriptive statistics from the mtcars dataset",
                                tfoot = "&dagger; The weight is in 10<sup>3</sup> kg"))
Basic descriptive statistics from the mtcars dataset
Automatic
No. 19 		Manual
No. 13
Gas 17 (15 - 19) 23 (21 - 30)
Weight† 4 (3 - 4) 2 (2 - 3)
Color
  black 8 (42%) 3 (23%)
  red 5 (26%) 4 (31%)
  silver 6 (32%) 6 (46%)
Gears
  3 15 (79%) 0 (0%)
  4 4 (21%) 8 (62%)
  5 0 (0%) 5 (38%)
† The weight is in 103 kg

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