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

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Introduction

This vignette introduces the following functions from the PHEindicatormethods package and provides basic sample code to demonstrate their execution. The code included is based on the code provided within the 'examples' section of the function documentation. This vignette does not explain the methods applied in detail but these can (optionally) be output alongside the statistics or for a more detailed explanation, please see the references section of the function documentation.

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

library(PHEindicatormethods)
library(dplyr)

Package functions

This vignette covers the following core functions available within PHEindicatormethods:

| Function | Type | Description | |:------------------|:--------------------------|:--------------------------------------------| | phe_proportion | Non-aggregate | Performs a calculation on each row of data (unless data is grouped) | | phe_rate | Non-aggregate | Performs a calculation on each row of data (unless data is grouped) | | | | | | phe_mean | Aggregate | Performs a calculation on each grouping set | | | | | | calculate_dsr | Aggregate, standardised | Performs a calculation on each grouping set and requires additional reference inputs | | calculate_ISRatio | Aggregate, standardised | Performs a calculation on each grouping set and requires additional reference inputs | | calculate_ISRate | Aggregate, standardised | Performs a calculation on each grouping set and requires additional reference inputs |

Other functions are introduced in separate vignettes.

Non-aggregate functions

Create some test data for the non-aggregate functions

The following code chunk creates a data frame containing observed number of events and populations for 4 geographical areas over 2 time periods that is used later to demonstrate the PHEindicatormethods package functions:

``` {r create test data1} df <- data.frame( area = rep(c("Area1","Area2","Area3","Area4"), 2), year = rep(2015:2016, each = 4), obs = sample(100, 2 * 4, replace = TRUE), pop = sample(100:200, 2 * 4, replace = TRUE)) df

#### Execute phe_proportion and phe_rate

**INPUT:** The phe_proportion and phe_rate functions take a single data frame as input with columns representing the numerators and denominators for the statistic.  Any other columns present will be retained in the output.

**OUTPUT:** The functions output the original data frame with additional columns appended.  By default the additional columns are the proportion or rate, the lower 95% confidence limit, the upper 95% confidence limit, the confidence level, the statistic name and the method.

**OPTIONS:** The functions also accept additional arguments to specify the level of confidence, the multiplier and a reduced level of detail to be output. 


Here are some example code chunks to demonstrate these two functions and the arguments that can optionally be specified

```r
# default proportion
phe_proportion(df, obs, pop)

# specify confidence level for proportion
phe_proportion(df, obs, pop, confidence = 99.8)

# specify multiplier to output proportions as percentages
phe_proportion(df, obs, pop, multiplier = 100)

# specify multiplier for proportion, confidence level and remove metadata columns
phe_proportion(df, obs, pop, confidence = 99.8, multiplier = 100, type = "standard")

# default rate
phe_rate(df, obs, pop)

# specify multiplier for rate and confidence level
phe_rate(df, obs, pop, confidence = 99.8, multiplier = 100)

# specify multiplier for rate, confidence level and remove metadata columns
phe_rate(df, obs, pop, type = "standard", confidence = 99.8, multiplier = 100)

These functions can also return aggregate data if the input dataframes are grouped:

# default proportion - grouped
df %>%
  group_by(year) %>%
  phe_proportion(obs, pop)

# default rate - grouped
df %>%
  group_by(year) %>%
  phe_rate(obs, pop)

Aggregate functions

The remaining functions aggregate the rows in the input data frame to produce a single statistic. It is also possible to calculate multiple statistics in a single execution of these functions if the input data frame is grouped - for example by indicator ID, geographic area or time period (or all three). The output contains only the grouping variables and the values calculated by the function - any additional unused columns provided in the input data frame will not be retained in the output.

The df test data generated earlier can be used to demonstrate phe_mean:

Execute phe_mean

INPUT: The phe_mean function take a single data frame as input with a column representing the numbers to be averaged.

OUTPUT: By default, the function outputs one row per grouping set containing the grouping variable values (if applicable), the mean, the lower 95% confidence limit, the upper 95% confidence limit, the confidence level, the statistic name and the method.

OPTIONS: The function also accepts additional arguments to specify the level of confidence and a reduced level of detail to be output.

Here are some example code chunks to demonstrate the phe_mean function and the arguments that can optionally be specified

# default mean
phe_mean(df,obs)

# multiple means in a single execution with 99.8% confidence
df %>%
    group_by(year) %>%
        phe_mean(obs, confidence = 0.998)

# multiple means in a single execution with 99.8% confidence and data-only output
df %>%
    group_by(year) %>%
        phe_mean(obs, type = "standard", confidence = 0.998)

Standardised Aggregate functions

Create some test data for the standardised aggregate functions

The following code chunk creates a data frame containing observed number of events and populations by age band for 4 areas, 5 time periods and 2 sexes:

``` {r create test data2} df_std <- data.frame( area = rep(c("Area1", "Area2", "Area3", "Area4"), each = 19 * 2 * 5), year = rep(2006:2010, each = 19 * 2), sex = rep(rep(c("Male", "Female"), each = 19), 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), obs = sample(200, 19 * 2 * 5 * 4, replace = TRUE), pop = sample(10000:20000, 19 * 2 * 5 * 4, replace = TRUE)) head(df_std)

#### Execute calculate_dsr
**INPUT:** The minimum input requirement for the calculate_dsr function is a single data frame with columns representing the numerators and denominators and standard populations for each standardisation category. The standard populations must be appended to the input data frame by the user prior to execution of the function. The 2013 European Standard Population is provided within the package in vector form (`esp2013`), which you can join to your dataset. Alternative standard populations can be used but must be provided by the user.  

**OUTPUT:** By default, the function outputs one row per grouping set containing the grouping variable values, the total count, the total population, the dsr, the lower 95% confidence limit, the upper 95% confidence limit, the confidence level, the statistic name and the method. 

**OPTIONS:** The function also accepts additional arguments to specify the level of confidence, the multiplier and a reduced level of detail to be output. It is also possible to calculate CIs when we can't assume events are independent - further details can be found in the DSR vignette.

Here are some example code chunks to demonstrate the calculate_dsr function and the arguments that can optionally be specified

```r

# Append the standard populations to the data frame
# calculate separate dsrs for each area, year and sex
df_std %>%
    mutate(refpop = rep(esp2013, 40)) %>%
    group_by(area, year, sex) %>%
    calculate_dsr(obs,pop, stdpop = refpop)


# Append the standard populations to the data frame
# calculate separate dsrs for each area, year and sex and drop metadata fields from output
df_std %>%
    mutate(refpop = rep(esp2013, 40)) %>%
    group_by(area, year, sex) %>%
    calculate_dsr(obs,pop, stdpop = refpop, type = "standard")

# calculate for under 75s by filtering out records for 75+ from input data frame and standard population
df_std %>%
  filter(ageband <= 70) %>%
  mutate(refpop = rep(esp2013[1:15], 40)) %>%
  group_by(area, year, sex) %>%
  calculate_dsr(obs, pop, stdpop = refpop)

Execute calculate_ISRatio and calculate_ISRate

INPUT: These functions take a single data frame as input, with columns representing the numerators and denominators for each standardisation category, plus reference numerators and denominators for each standardisation category.

The reference data can either be provided in a separate data frame/vectors or as columns within the input data frame:

• reference data provided as a data frame or as vectors - the data frame/vectors and the input data frame must both contain rows for the same standardisation categories, and both must be sorted, within each grouping set, by these standardisation categories in the same order.

• reference data provided as columns within the input data frame - the reference numerators and denominators can be appended to the input data frame prior to execution of the function - if the data is grouped to generate multiple indirectly standardised rates or ratios then the reference data will need to be repeated and appended to the data rows for every grouping set.

OUTPUT: By default, the functions output one row per grouping set containing the grouping variable values, the observed and expected counts, the reference rate (ISRate only), the indirectly standardised rate or ratio, the lower 95% confidence limit, and the upper 95% confidence limit, the confidence level, the statistic name and the method.

OPTIONS: If reference data are being provided as columns within the input data frame then the user must specify this as the function expects vectors by default. The function also accepts additional arguments to specify the level of confidence, the multiplier and a reduced level of detail to be output.

The following code chunk creates a data frame containing the reference data - this example uses the all area data for persons in the baseline year:

``` {r create reference data} df_ref <- df_std %>% filter(year == 2006) %>% group_by(ageband) %>% summarise(obs = sum(obs), pop = sum(pop), .groups = "drop_last")

head(df_ref)

Here are some example code chunks to demonstrate the calculate_ISRatio function and the arguments that can optionally be specified

```r
# calculate separate smrs for each area, year and sex
# standardised against the all-year, all-sex, all-area reference data
df_std %>%
    group_by(area, year, sex) %>%
    calculate_ISRatio(obs, pop, df_ref$obs, df_ref$pop)

# calculate the same smrs by appending the reference data to the data frame
# and drop metadata columns from output
df_std %>%
    mutate(refobs = rep(df_ref$obs,40),
           refpop = rep(df_ref$pop,40)) %>%
    group_by(area, year, sex) %>%
    calculate_ISRatio(obs, pop, refobs, refpop, refpoptype = "field",
                      type = "standard")

The calculate_ISRate function works exactly the same way but instead of expressing the result as a ratio of the observed and expected rates the result is expressed as a rate and the reference rate is also provided. Here are some examples:

# calculate separate indirectly standardised rates for each area, year and sex
# standardised against the all-year, all-sex, all-area reference data
df_std %>%
    group_by(area, year, sex) %>%
    calculate_ISRate(obs, pop, df_ref$obs, df_ref$pop)

# calculate the same indirectly standardised rates by appending the reference data to the data frame
# and drop metadata columns from output
df_std %>%
    mutate(refobs = rep(df_ref$obs,40),
           refpop = rep(df_ref$pop,40)) %>%
    group_by(area, year, sex) %>%
    calculate_ISRate(obs, pop, refobs, refpop, refpoptype = "field",
                     type = "standard")

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