    library(chmsflow)

## Introduction

There are two types of derived variables in the CHMS surveys. Both are supported in chmsflow.

- Variable mapping – mapping two or more variables into a single variable.
- Computed variables – variables derived using mathematical equations or clinical logic.

chmsflow computes derived variables using functions referenced in `variable-details.csv`. The `recEnd` column uses the prefix `Func::` to name the R function, and the `variableStart` column uses the prefix `DerivedVar::` to list the input variables.

For example, GFR (`gfr_ml_min`) has:

- `recEnd`: `Func::calculate_gfr`
- `variableStart`: `DerivedVar::[lab_bcre, pgdcgt, clc_sex, clc_age]`

This tells `rec_with_table()` to call `calculate_gfr()` with the four input variables.

## How to use derived variables

Since derived variables depend on their input variables, you must list both the derived variable and its inputs when calling `rec_with_table()`:

    cycle2_gfr <- recodeflow::rec_with_table(
      cycle2,
      variables = c("lab_bcre", "pgdcgt", "clc_sex", "clc_age", "gfr_ml_min"),
      variable_details = variable_details,
      log = TRUE
    )

For variables that depend on medication status (e.g., hypertension, diabetes), use `recode_after_meds()` instead of `rec_with_table()`. See Recoding medications and Analysis walkthrough for the full workflow.

## Creating a derived variable

To add a new derived variable to chmsflow, you need to create a harmonized set of input variables and an R function that computes the derived value. See How to add variables for step-by-step instructions.

For details on the metadata schema, see Variable schema reference.

## Next steps

- See derived variables in a full analysis – The Analysis walkthrough demonstrates deriving hypertension status from CHMS cycle 3 data.
- Handle missing data – Learn how `tagged_na()` codes propagate through derived variable functions in Missing data (tagged_na).
- Understand the methodology – For the design rationale behind the rules-as-data approach, see Methodology.