In publichealthengland/PHEindicatormethods: Common Public Health Statistics and their Confidence Intervals

knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>"
)

Introduction

This vignette documents the method for calculating DSRs and their confidence limits using the PHEindicatormethods::calculate_dsr() function. The function calculates confidence limits using the Dobson method, with an option to adjust confidence intervals for non-independent events.

The function can be used to calculate DSRs for multiple groups. For example, DSRs for multiple geographic areas, sexes, time periods and/or indicators can be calculated in a single execution. It takes the following arguments as inputs:

| Argument | Type | Definition |Default value | |:-------------------|:-----------------|:-----------------------------------------------------------------------|:--------------| | data | data frame | data.frame containing the data to be standardised | none | | x | unquoted string | field name from data containing the observed number of events for each standardisation category (eg ageband) within each grouping set (eg area or indicator) | none | | n | unquoted string | field name from data containing the populations for each standardisation category (eg ageband) within each grouping set (eg area or indicator)| none | | stdpop | unquoted string | field name from data containing the standard populations for each age group | NULL | | type | quoted string | defines the data and metadata columns to include in output. Can by 'value', 'lower', 'upper', 'standard' or 'full' | "full" | | confidence | numeric value | the required level of confidence expressed as a number between 0.9 and 1 or 90 and 100 | 0.95 | | multiplier | numeric value | the multiplier used to express the final values (eg 100,000 = rate per 100,000 | 100,000 | | independent_events | boolean | whether events are independent | TRUE | | eventfreq | unquoted string | field name from data containing the event frequencies | NULL | | ageband | unquoted string | field name form data containing the age bands for standardisation | NULL |

Note that the European Standard Population 2013 divided into 19 five-year agebands (0-4, 5-9, 10-14, .....90+) is provided in vector format within the package. You can join this to your dataset to create a standard population column prior to calling calculate_dsr.

If multiple DSRs are required from a single data frame then the data frame must be grouped prior to inputting to the function - this is demonstrated below.

The following packages must be installed and loaded if not already available

library(PHEindicatormethods)
library(dplyr)
library(tidyr)

First let's create some data to play with

In a real situation we'd most likely be sourcing our numerators and denominators from different places so let's create them separately for now.

pops <- data.frame(
  indicator = rep(c("Ind1", "Ind2", "Ind3", "Ind4"), each = 19 * 2 * 5),
  period    = rep(2012:2016, each = 19 * 2, times = 4),
  region    = rep(rep(c("Area1", "Area2"), each = 19), times = 20),
  ageband   = rep(c(0,  5, 10, 15, 20, 25, 30, 35, 40, 45, 50,
                   55, 60, 65, 70, 75, 80, 85, 90), times = 40),
  pop       = sample(10000:20000, 19 * 2 * 5 * 4, replace = TRUE),
  esp2013   = rep(esp2013, times = 40))
head(pops)


deaths <- data.frame(
  indicator = rep(c("Ind1", "Ind2", "Ind3", "Ind4"), each = 19 * 2 * 5),
  period = rep(2012:2016, each = 19 * 2),
  region = rep(rep(c("Area1", "Area2"), each = 19), times = 5),
  ageband = rep(c(0,  5, 10, 15, 20, 25, 30, 35, 40, 45, 50,
                 55, 60, 65, 70, 75, 80, 85, 90), times = 10),
  dths = sample(200, 19 * 2 * 5 * 4, replace = TRUE))
head(deaths)

Our data contains records for 4 different indicators, 5 time periods and 2 geographies so let's calculate a DSR for each combination - that's 40 separate DSRs from a single execution of the calculate_dsr function......

Prepare the data frame

First we'll need to join our datasets to create the input data frame for the function. We also need to specify our grouping sets:

``` {r create reference column} df <- left_join(pops, deaths, by = c("indicator", "period", "region", "ageband")) %>% group_by(indicator, period, region)

## Now let's calculate some DSRs

By default the function will apply 95% confidence intervals, a 100,000
multiplier and will output 3 data fields against each grouping set:

* the dsr value  
* the lower confidence limit
* the upper confidence limit  

It will also output 3 metadata fields as an audit showing which argument
parameters were passed:

* confidence - the confidence level(s) returned
* statistic - the statistic including the multiplier applied
* method - the DSR method applied

``` {r calculate DSRs}
calculate_dsr(
  data = df, 
  x = dths, # name of field containing count of events
  n = pop, # name of field containing population denominators
  stdpop = esp2013 # name of field containing standard populations
)

Alternatively, we can drop metadata fields by specifying the 'type' argument value as 'standard', and adjust the multiplier applied to the DSR:

``` {r alternative dsr} calculate_dsr( data = df, x = dths, n = pop, stdpop = esp2013, type = "standard", confidence = 99.8, multiplier = 10000 )

</br>
</br>

## Alternative Standard Populations

In some cases you may wish to standardise against a different population to the
default esp2013 one provided - such as the 1976 European Standard Population or
an age and sex standardised population.  This can be done by appending the
required standard populations to your data frame before executing the function.

In the example below the data we have used previously are duplicated for males
and females and then different standard populations are applied to each gender.
For the purposes of the example, dummy standard populations have been used which
very crudely represent more women living to the oldest age groups than men -
these are not from any official source and should not be used in real analysis.
Notice that despite the data counts and populations being the same for males and
females the different standard populations used result in different DSRs being
produced.


``` {r specify stdpop as field name}
# duplicate data for males and females and apply different standard populations
# to each sex
df_f <- df %>%
  mutate(
    sex = "F",
    esp_dummy = c(5000, 5500, 5500, 5500, 6000, 6000, 6500, 7000, 7000, 7000,
                  7000, 6500, 5500, 5000, 4500, 4000, 3000, 2000, 1500)
  )

df_m <- df %>%
  mutate(
    sex = "M",
    esp_dummy = c(5000, 5500, 5500, 5500, 6000, 6000, 6500, 7000, 7000, 7000,
                  7000, 6500, 6500, 6000, 5500, 4000, 2000, 1000, 500)
  )

df_mf <- df_f %>%
  bind_rows(df_m) %>%
  group_by(sex, .add = TRUE) %>%
  select(!"esp2013")

# add sex to the grouping variables then calculate the DSRs
dsrs_mf <- calculate_dsr(
  df_mf, 
  x = dths, 
  n = pop, 
  stdpop = esp_dummy
)

head(dsrs_mf)

Calculating DSRs when the events are non-independent

Methodological advice on calculating directly standardised rates when observed events are not independent can be found in the DSR chapter of the Fingertips Public Health Technical Guidance.

This method adjusts the confidence intervals around the DSR by considering the frequency of events per individual. In the example below, event frequency data is added to the input data frame and the calculate_dsr function is then applied with the independent_events argument set to FALSE. The event frequency and age band column names are additionally passed into the function. Note that the DSRs output are identical to those produced for the df data frame at the beginning of this vignette, but the 95% confidence intervals are wider.

# Generate some dummy data
# breakdown original dataset to show event frequencies and to count unique individuals
df_freq <- df %>%
 mutate(
   f3 = floor((dths * 0.1) / 3),           # 10% of events in individuals with 3 events
   f2 = floor((dths * 0.2) / 2),           # 20% of events in individuals with 2 events
   f1 = (dths - (3 * f3) - (2 * f2))) %>%  # 70% of events in individuals with 1 event
 select(!"dths") %>%
 pivot_longer(
   cols = c("f1", "f2", "f3"),
   names_to = "eventfrequency",
   values_to = "uniqueindividuals",
   names_prefix = "f") %>%
   mutate(eventfrequency = as.integer(eventfrequency)
 )

# calculate the dsrs - notice that output DSRs values match those calculated
# earlier for the same data frame but confidence intervals are wider
df_freq %>%
  group_by(eventfrequency, .add = TRUE) %>%
  calculate_dsr(
    x = uniqueindividuals, # count of unique individuals experiencing the frequency of events in eventfreq
    n = pop,
    stdpop = esp2013,
    independent_events = FALSE, # calculate CIs assuming events are not independent
    eventfreq = eventfrequency, # name of column containing the event frequencies (e.g. 1, 2, ...)
    ageband = ageband # name of column containing age bands
  )

Calculating DSRs when there are zero deaths in some age bands

This is a fairly common scenario, especially when working with small populations where there may be no deaths in some of the younger age groups. The calculate_dsr function can handle this scenario and will assume a zero death count where it is missing or recorded as NA.

Let's create a couple of data frames to demonstrate this. In this example, there are no deaths in the 10-14, 15-20 and 20-14 age bands. If we join these data frames to produce the input data frame required for the calculate_dsr function then we get NA values in the Deaths column.

``` {r test data} pops2 <- data.frame( ageband = c( 0, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90), pop = c(30, 35, 35, 35, 40, 40, 45, 50, 50, 50, 60, 60, 70, 75, 70, 60, 20, 20, 15), esp2013 = esp2013 )

deaths2 <- data.frame( ageband = c(0, 5, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90), dths = c(1, 1, 1, 1, 3, 3, 3, 3, 10, 10, 10, 10, 8, 8, 8, 8) )

df2 <- left_join(pops2, deaths2, by = "ageband")

head(df2)

calculate_dsr( df2, x = dths, n = pop, stdpop = esp2013 ) ```

publichealthengland/PHEindicatormethods documentation built on Dec. 15, 2024, 3:18 p.m.