Working with CHI numbers
In phsmethods: Standard Methods for Use in Public Health Scotland

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

Checking and correcting CHI numbers

The first thing you should do when working with CHI numbers of unknown quality is to check their validity. phsmethods provides a function to easily do this: chi_check() - This function expects a character vector (1 or more), it will then return a value for each CHI number letting you know if it's valid, and if it isn't what the issue is.

chi_numbers <- c(
  "0211165794",
  "9999999999",
  "402070763",
  "00402070763",
  "0101010000",
  "Missing CHI",
  NA,
  ""
)

library(phsmethods)

chi_check(chi_numbers)

Cleaning up bad CHI numbers

Usually, we will have the CHI as a variable in some data.

library(dplyr)

data <- tibble(
  chi = c(
    "0211165794",
    "9999999999",
    "402070763",
    "00402070763",
    "0101010000",
    "Missing CHI",
    NA,
    ""
  )
)

It looks like one of the CHI numbers '402070763' might have just lost a leading zero, this is a common occurrence if the data has passed through Excel at some point. We can fix this specific issue with chi_pad().

fixed_data <- data %>%
  mutate(chi = chi_pad(chi))

checked_data <- fixed_data %>%
  mutate(valid_chi = chi_check(chi))

checked_data

On a larger dataset, it might be useful to get a count of the issues rather than seeing them per CHI.

fixed_data %>%
  count(valid_chi = chi_check(chi), sort = TRUE)

Now we have this knowledge we have a few options. Which option we take will depend on the type and purpose of the analysis as well as how many CHI numbers have issues.

Go back to the data source/provider and try to fix the erroneous CHI numbers.
Set them as NA so we keep the rest of the data.
Filter the data with invalid CHIs out completely.

fixed_data %>%
  mutate(chi = if_else(chi_check(chi) != "Valid CHI", NA_character_, chi))

fixed_data %>%
  filter(chi_check(chi) == "Valid CHI")

Inferring data from a CHI number

In an ideal world, we would always have supplementary data such as Date of Birth, Age and Sex alongside the CHI number, however, we often work with data where we only have the CHI number and the other demographic variables are either completely missing or incomplete.

Once we have checked and (if necessary) padded the CHI numbers, we can then try and extract some information from them.

The structure of a CHI number

As explained in this Wikipedia article{target="_blank"} a CHI number is constructed as follows:

The first 6 digits are the patient's Date of Birth in the format DDMMYY.
Digits 7 and 8 are random.
The 9^th number indicates the patient's sex - odd for male, even for female.
The final, 10^th, digit is a 'check digit' that is calculated from the preceding 9 digits - This helps guard against transcription errors, as it is unlikely that an incorrect number will still produce a valid check digit. Historically, CHIs have used a Modulus-11 (Mod-11) check digit; however, from August 2026 new CHI numbers may instead use either a Mod‑11 or a Modulus-10 (Mod‑10) (Luhn) check digit.

Modulus-11 check

While chi_check() performs validation of check digits in CHI numbers for you, it may be of interest to understand the details behind how it works. For Mod-11, the steps are:

Step 1: Multiply each of the first nine digits of the CHI number by a weighting factor (10:2), i.e. multiply the first digit by 10, multiply the second digit by 9,... , multiply the ninth digit by 2.
Step 2: Sum up the results from step 1.
Step 3: Divide the total from step 2 by 11 and derive the remainder.
Step 4: Subtract the remainder derived in step 3 from 11 (i.e. calculate 11 minus remainder).
Step 5: If the result from step 4 is 11, set it to zero; otherwise leave the result as it is.
Step 6: Check that the result from step 5 matches the tenth digit of the CHI number. If it does not match then the CHI is not valid.

Changes to CHI numbers (August 2026): Inclusion of Modulus-10 check

There is a limit to the number of possible CHI numbers that can be generated for each combination of date of birth and sex. This limit is being reached for CHI numbers that involve dates of birth on 1^st January for certain years. To increase the limit of possible CHI numbers that can be generated, the current rules (in 2025) which require CHI numbers to have a valid Mod-11 check digit are being made less restrictive. Specifically, from August 2026, changes are being implemented that will allow CHI numbers to be assigned to patients if they have a valid Mod-11 or Mod-10{target="_blank"} check digit. This change means that there are a range of scenarios that a CHI number can fit:

CHI passes Mod-11 but fails Mod-10.
CHI passes Mod-11 and passes Mod-10.
CHI fails Mod-11 but passes Mod-10.
CHI fails Mod-11 and fails Mod-10.

For scenarios 1, 2, and 4, the results of the CHI check will remain the same as before. However, permitting scenario 3 will result in more CHI numbers passing the CHI check. It is important to note that prior to the implementation date that allows Mod-10 to be used for CHI numbers, no individuals will have been assigned a CHI number that passes Mod-10 but fails Mod-11.

By default, chi_check() will output as "Valid CHI" if CHI numbers pass either a Mod-11 or Mod-10 check. Therefore, the default setting for chi_check() can allow some CHI numbers to be returned as a valid CHI when they are not. For instance, this can happen if you know a dataset only contains CHI numbers assigned to patients prior to the Mod-10 implementation date. In this case, you can use chi_check(chi, check_mod10 = FALSE) to apply a CHI check that only allows CHI numbers that pass a Mod-11 check to be returned as valid.

If a dataset contains data where it is possible for patients to have been assigned a CHI number which passes Mod-10 (i.e. patient was first registered after the Mod-10 implementation and had an affected date of birth), then a small error rate may occur when applying the CHI check function with default settings. While this error rate is very small, one approach to reduce this error rate is to apply the CHI check conditionally. For example, a Mod-10/Mod-11 check can be applied only to CHI numbers that have dates of birth starting on 1^st January and the Mod-11 check can be used otherwise. If you want to apply a check following that logic, if_else() or case_when() can be used.

fixed_data %>%
  mutate(
    chi_valid = if_else(
      stringr::str_detect(chi, "^0101"),
      chi_check(chi),
      chi_check(chi, check_mod10 = FALSE)
    )
  )

Extracting sex from CHI

With sex_from_chi() we can extract the infer and extract the patient's sex. By default, the function will first check the CHI for validity and will return NA if a CHI is invalid. Note that from August 2026, this check will allow options to check CHI against Mod-11 and/or Mod-10 (the same applies when checking CHIs within the functions to extract date of birth or age from CHI).

If you have already checked the CHI in a previous step it can be useful to use chi_check = FALSE as this will be faster.

data <- tibble(
  chi = c(
    "0101011237",
    "0211165794",
    "0402070763",
    "0101336489",
    "1904851231",
    "2902960018"
  )
)

# Confirm all of the CHIs are valid
count(data, chi_check(chi, check_mod10 = FALSE))

data_sex <- data %>%
  mutate(sex = sex_from_chi(chi, chi_check = FALSE))
data_sex

By default sex will be returned as an integer with '1' representing 'Male' and '2' representing 'Female', this is consistent with the coding of sex in other PHS datasets.

We can have sex returned as a factor using as_factor = TRUE, which by default will have levels of '1' and '2' and labels of 'Male' and 'Female' which can be useful, particularly when visualising the data.

data_sex <- data_sex %>%
  mutate(sex_factor = sex_from_chi(chi, as_factor = TRUE))

data_sex

library(ggplot2)

data_sex %>%
  ggplot(aes(y = "", fill = sex_factor)) +
  geom_bar() +
  coord_polar() +
  labs(title = "Count of Male vs Female", x = "", y = "") +
  scale_fill_brewer("Sex (from CHI)", type = "qual") +
  theme_minimal()

Extracting Date of Birth from CHI

It is usually not possible to definitively infer a patient's Date of Birth from the CHI number, this is because the CHI only contains 2 digits for the year. Looking at the first 6 digits of a CHI number '010120' could be '1 January 1920' or '1 January 2020'. However, with some extra context, we can usually eliminate one of the possibilities, for example in 2023 we know that any CHI numbers of the form 'DDMM24' etc. must mean 1924 since it can't be 2024.

The function dob_from_chi() will try to extract the Date of Birth and will return NA if the date is ambiguous.

data_dob <- data %>%
  mutate(dob = dob_from_chi(chi))

data_dob

We will need to provide some more context to be able to work out the still missing dates. Often we will be working with historical data, for instance, if we know the data is from 2015 we know the patients must have been born earlier than that. We can use the min_date and max_date arguments to provide this context.

min_date will usually be some information from the data, it is the latest possible date that the CHI could have been born.
max_date will default to today's date, it will usually be some common sense date about the latest date you'd expect. For instance, if working with childhood vaccine data you could use Sys.Date() - lubridate::years(16), it to imply you don't expect anyone older than 16 as of today's date.

# Expect no one born after 2015-12-31
data %>%
  mutate(dob = dob_from_chi(chi, max_date = as.Date("2015-12-31")))

# Expect no one born before 1999-12-31 i.e. 16 years before our data started.
data %>%
  mutate(
    dob = dob_from_chi(
      chi,
      max_date = as.Date("2015-12-31"),
      min_date = as.Date("2015-12-31") - lubridate::years(16)
    )
  )

Usually, we will have event dates e.g. an admission date alongside the data and this can be used instead of, or in conjunction with a fixed date.

data <- data %>%
  mutate(
    event_date = as.Date(c(
      "2015-01-01",
      "2014-01-01",
      "2013-01-01",
      "2012-01-01",
      "2011-01-01",
      "2010-01-01"
    ))
  )

# Using the event date as the maximum date
data %>%
  mutate(dob = dob_from_chi(chi, max_date = event_date))

# Setting a 'fixed' minimum date as well as using the event date
data_dob <- data %>%
  mutate(
    dob = dob_from_chi(
      chi,
      max_date = event_date,
      min_date = as.Date("1915-01-01")
    )
  )

data_dob

Extracting age from CHI

The function age_from_chi() provides a simpler interface for just extracting a patient's age from the CHI number. In the background, it uses dob_from_chi() but allows you to specify min_age and max_age, which are usually conceptually simpler than trying to work out dates. We do lose some amount of fine control here though, so it will sometimes be necessary to use dob_from_chi() and then age_calculate().

Note that age is calculated at today's date unless otherwise specified with the ref_date argument.

data %>%
  mutate(age = age_from_chi(chi))

# Work out age at a fixed date
data %>%
  mutate(age = age_from_chi(chi, ref_date = as.Date("2016-01-01")))

# Work out age at a relative date
data %>%
  mutate(age = age_from_chi(chi, ref_date = event_date))

We will get different results depending on which context we supply.

data %>%
  mutate(age = age_from_chi(chi, ref_date = event_date, max_age = 18))

data %>%
  mutate(
    age = age_from_chi(
      chi,
      ref_date = event_date,
      min_age = 60,
      max_age = 120
    )
  )

data %>%
  mutate(
    age = age_from_chi(
      chi,
      min_age = 60,
      max_age = 120
    )
  )