Birth-weight case study: from data to Table 1 • gtstats

From raw clinical codes to a report-ready table

This case study shows the intended gtstats workflow. A maternity team wants to describe participants by low-birth-weight outcome, understand the continuous variables, and produce a Table 1. The example uses the labelled birth-weight teaching dataset included with gtstats.

1. Prepare readable data

library(gtstats)
data("birthwt", package = "gtstats")
birthwt_data <- birthwt

The built-in data keep the original code names but supply readable variable labels and factors. antenatal_visits is explicitly ordered; numeric clinical codes are not silently treated as ordinal merely because their values are ordered.

2. Start with the data, not a test

overview <- describe_data(birthwt_data)
to_flextable(overview)

Variable	Type	Complete	Unique	Overview	Range / levels
Birth-weight outcome [low]	binary	189/189 (100.0%)	2	Normal birth weight 130 (68.8%); Low birth weight 59 (31.2%)	Normal birth weight, Low birth weight
Maternal age (years) [age]	continuous	189/189 (100.0%)	24	Mean 23.24 (SD 5.30); median 23.00	14.00 to 45.00
Maternal weight (lb) [lwt]	continuous	189/189 (100.0%)	75	Mean 129.81 (SD 30.58); median 121.00	80.00 to 250.00
Maternal race [race]	categorical	189/189 (100.0%)	3	White 96 (50.8%); Other 67 (35.4%); Black 26 (13.8%)	White, Black, Other
Smoking during pregnancy [smoke]	binary	189/189 (100.0%)	2	No 115 (60.8%); Yes 74 (39.2%)	No, Yes
Previous premature labours [ptl]	categorical*	189/189 (100.0%)	4	0 159 (84.1%); 1 24 (12.7%); 2 5 ( 2.6%)	0, 1, 2, 3
Hypertension [ht]	binary	189/189 (100.0%)	2	No 177 (93.7%); Yes 12 ( 6.3%)	No, Yes
Uterine irritability [ui]	binary	189/189 (100.0%)	2	No 161 (85.2%); Yes 28 (14.8%)	No, Yes
First-trimester visits [ftv]	continuous	189/189 (100.0%)	6	Mean 0.79 (SD 1.06); median 0.00	0.00 to 6.00
Birth weight (g) [bwt]	continuous	189/189 (100.0%)	131	Mean 2944.59 (SD 729.21); median 2977.00	709.00 to 4990.00
Previous premature labour [previous_preterm]	binary	189/189 (100.0%)	2	No 159 (84.1%); Yes 30 (15.9%)	No, Yes
First-trimester visits [antenatal_visits]	ordinal	189/189 (100.0%)	3	None 100 (52.9%); One 47 (24.9%); Two or more 42 (22.2%)	None, One, Two or more
One row is shown per selected variable.
Potential data-quality findings and interpretation prompts are stored in `$issues`.
* Possible ordinal or count-coded variable; confirm the intended meaning and order from the data dictionary or clinical context.

overview$issues
#> # A tibble: 2 × 5
#>   variable label                      issue          why_flagged suggested_check
#>   <chr>    <chr>                      <chr>          <chr>       <chr>          
#> 1 ptl      Previous premature labours Sparse catego… At least o… Confirm coding…
#> 2 ftv      First-trimester visits     Low-cardinali… Only 6 dis… Confirm whethe…

describe_data() is the first pass. It reports type, completeness, levels or range, and a short type-specific overview. It does not decide a statistical test.

3. Assess selected continuous variables

distribution <- assess_distribution(
  birthwt_data,
  vars = c(age, lwt),
  by = low
)
to_flextable(distribution)

Variable	Group	n	Skewness	Shape	Suggested presentation	Shapiro p
Maternal age (years)	Normal birth weight	130	0.74	Some right asymmetry	Review mean (SD) and median (IQR)	<0.001
	Low birth weight	59	0.29	Little/no asymmetry		0.521
Maternal weight (lb)	Normal birth weight	130	1.42	Marked right skew	Median (IQR) preferred	<0.001
	Low birth weight	59	1.06	Marked right skew		<0.001
Shape categories use absolute sample skewness: little/no asymmetry < 0.50; some asymmetry 0.50 to < 1.00; marked skew >= 1.00. They are descriptive guidance, not formal classifications.
Shapiro-Wilk is sensitive to sample size. Interpretation should consider skewness, graphical assessment, sample size and subject-matter knowledge.
Suggested summaries are intended for descriptive reporting only and should not be used alone to determine inferential statistical methods.
For grouped data, the suggested presentation applies to all groups of each variable.

distribution$recommendations
#> # A tibble: 2 × 5
#>   variable label                overall_recommendation            reason  review
#>   <chr>    <chr>                <chr>                             <chr>   <chr> 
#> 1 age      Maternal age (years) Review mean (SD) and median (IQR) Some a… Inspe…
#> 2 lwt      Maternal weight (lb) Median (IQR) preferred            Marked… Inspe…

The first table retains group-specific diagnostics. The recommendation table gives one reporting approach per variable, so the same summary can be used across Table 1 groups. Shapiro-Wilk is supporting information, not a rule for choosing an inferential test. Use plots = TRUE when a histogram, density, Q-Q plot, and boxplot are useful.

assess_variance() provides the related spread diagnostic. It reports group SDs, variances, and ratios without using a variance test to gatekeep Welch methods.

variance <- assess_variance(birthwt_data, vars = c(age, lwt), by = low)
to_flextable(variance)

Variable	Normal birth weight	Low birth weight	Observed SD ratio	Observed variance ratio	Levene p	Interpretation
Maternal age (years)	n = 130; SD = 5.58; variance = 31.19	n = 59; SD = 4.51; variance = 20.35	1.24	1.53	0.102	Levene found no clear evidence of different spreads; this does not prove equal variances.
Maternal weight (lb)	n = 130; SD = 31.72; variance = 1006.41	n = 59; SD = 26.56; variance = 705.40	1.19	1.43	0.476	Levene found no clear evidence of different spreads; this does not prove equal variances.
SD and variance ratios are the largest group value divided by the smallest group value. They describe observed spread; they are not pass/fail tests.
For independent groups, Welch t-tests and Welch ANOVA do not require equal variances. `assess_variance()` does not select an inferential test and does not assess pairing or repeated-measures sphericity.
The displayed Levene test is median-centred (Brown-Forsythe). It is supporting information only: its p-value neither proves equal variances nor selects an inferential test.
Interpret spread alongside sample size, distributional shape, outliers, missingness, and the study design.

4. Build Table 1

table_one <- summary_table(
  birthwt_data,
  by = low,
  include = c(age, lwt, race, smoke, ht, ui,
              previous_preterm, antenatal_visits),
  overall = "first"
) |>
  add_p()

to_flextable(table_one)

Characteristic	Overall N = 189	Normal birth weight N = 130	Low birth weight N = 59	p-value
Maternal age (years)	23.2 (5.3)	23.7 (5.6)	22.3 (4.5)	0.078ᵃ
Maternal weight (lb)	121.0 (110.0–140.0)	123.5 (113.0–147.0)	120.0 (104.0–130.0)	0.013ᵇ
Maternal race				0.082ᶜ
White	96 (50.8%)	73 (56.2%)	23 (39.0%)
Black	26 (13.8%)	15 (11.5%)	11 (18.6%)
Other	67 (35.4%)	42 (32.3%)	25 (42.4%)
Smoking during pregnancy				0.040ᶜ
No	115 (60.8%)	86 (66.2%)	29 (49.2%)
Yes	74 (39.2%)	44 (33.8%)	30 (50.8%)
Hypertension				0.052ᵈ
No	177 (93.7%)	125 (96.2%)	52 (88.1%)
Yes	12 (6.3%)	5 (3.8%)	7 (11.9%)
Uterine irritability				0.035ᶜ
No	161 (85.2%)	116 (89.2%)	45 (76.3%)
Yes	28 (14.8%)	14 (10.8%)	14 (23.7%)
Previous premature labour				<0.001ᶜ
No	159 (84.1%)	118 (90.8%)	41 (69.5%)
Yes	30 (15.9%)	12 (9.2%)	18 (30.5%)
First-trimester visits				0.281ᶜ
None	100 (52.9%)	64 (49.2%)	36 (61.0%)
One	47 (24.9%)	36 (27.7%)	11 (18.6%)
Two or more	42 (22.2%)	30 (23.1%)	12 (20.3%)
Continuous data: Maternal age (years): mean (SD); Maternal weight (lb): median (IQR). Categorical data are n (%).
ᵃ Welch t-test; ᵇ Wilcoxon rank-sum test; ᶜ Chi-square test; ᵈ Fisher's exact test

This is already a publication-ready flextable. Use customise_table() to finish it for Word; use to_gt() only when a gt object is specifically needed.

table_one |>
  customise_table(
    title = "Participant characteristics by birth-weight outcome",
    theme = "journal"
  ) |>
  save_output("table-1.docx")

5. Ask a focused inferential question

weight_comparison <- compare_groups(
  birthwt_data,
  variable = lwt,
  group = low
)
to_flextable(weight_comparison)

Variable	Normal birth weight N = 130	Low birth weight N = 59	p-value
Maternal weight (lb)	123.50 (113.00–147.00)	120.00 (104.00–130.00)	0.01
The analysis assumes independent observations; study design must confirm this.
Automatic selection used distribution guidance or design/cell-count rules. Rule applied: Two-group continuous outcome: marked skewness flagged in at least one group; selected Wilcoxon rank-sum test.
Wilcoxon rank-sum is rank based; similar distribution shapes are needed for a median-shift interpretation.


# The exact automatic rule and observed inputs are retained for review.
weight_comparison$method$selection_rule
#> [1] "Two-group continuous outcome: marked skewness flagged in at least one group; selected Wilcoxon rank-sum test."
weight_comparison$method$selection_inputs
#> $distribution_guidance
#> [1] "Skewed; Skewed"
#> 
#> $skewness_flagged
#> [1] TRUE
#> 
#> $groups
#> [1] 2
#> 
#> $paired
#> [1] FALSE
#> 
#> $var_equal
#> [1] FALSE
#> 
#> $variance_assumption_source
#> [1] "User-specified; not inferred from a variance hypothesis test"
#> 
#> $observed_group_spread
#> $observed_group_spread$group_spread
#> # A tibble: 2 × 4
#>   group                   n    sd variance
#>   <chr>               <int> <dbl>    <dbl>
#> 1 Normal birth weight   130  31.7    1006.
#> 2 Low birth weight       59  26.6     705.
#> 
#> $observed_group_spread$sd_ratio
#> [1] 1.194461
#> 
#> $observed_group_spread$variance_ratio
#> [1] 1.426737

assumptions_stats(weight_comparison)

Variable	Check before reporting	Action	Details
Checks before reporting
Maternal weight (lb)	Independent observations	Confirm from study design	Confirm from the study design that each observation contributes independently.
Maternal weight (lb)	Comparable distribution shapes	Confirm from study design	Required when interpreting the rank-based result specifically as a location or median shift.

diagnostics_stats(weight_comparison)

Variable	Check	Result	Observed value	Reference	Interpretation
Diagnostics
Maternal weight (lb)	Comparison design	independent	Independent observations	Defined by the study design	Independent comparison; confirm independence from the study design.
Maternal weight (lb)	Variance assumption	welch default	var_equal = FALSE	User-specified analytical assumption	Welch is the conservative default and does not require equal variances. No variance hypothesis test was used.
Maternal weight (lb)	Automatic test selection	Wilcoxon rank-sum test	Skewed; Skewed	No marked group-level skewness flag for parametric default	Two-group continuous outcome: marked skewness flagged in at least one group; selected Wilcoxon rank-sum test.
Maternal weight (lb)	Distribution guidance	rank based recommended	Skewed; Skewed	Marked absolute skewness guidance; Shapiro-Wilk is supporting information only	Assessment applied within each group.
Maternal weight (lb)	Observed group spread	descriptive context	Normal birth weight (n = 130): SD 31.72; variance 1006.41; Low birth weight (n = 59): SD 26.56; variance 705.40; SD ratio = 1.19; variance ratio = 1.43	Descriptive diagnostic; no pass/fail threshold	Observed spread is descriptive only; no Levene, Bartlett, or F test was performed. `var_equal = FALSE` retains Welch methods as the conservative parametric auto default.

denominators_stats(weight_comparison)

Variable	Level	Group	Eligible observations	Used in analysis	Missing / excluded	Numerator	Denominator	Rule
Denominator audit
lwt	NA	low = Normal birth weight	130	130	0	NA	130	Non-missing outcome observations within group
lwt	NA	low = Low birth weight	59	59	0	NA	59	Non-missing outcome observations within group

compare_groups() answers one question at a time. The publication table is the result; the helper functions expose the assumptions, automatic checks, and analysis denominators for review. Use effect_size() separately when the magnitude of the difference is the main question.

What this workflow deliberately avoids

Selecting a test solely from a normality p-value.
Recommending a different descriptive summary in each group of the same Table 1 variable.
Treating every numeric code as continuous or ordinal without context.
Requiring manual formatting before a table can be shared.