This function, `toAnnual`, calculates the equivalent **annual churn rate** based...

This function, toAnnual, calculates the equivalent annual churn rate based on a given monthly churn rate. Here's a breakdown of how it works:

1. Input Normalization:

   • If the input monthlyChurn is greater than 1, the function assumes it is expressed as a percentage (e.g., 10 for 10%) rather than a decimal (e.g., 0.1 for 10%).
   • It divides the input by 100 to convert it into a decimal format.

2. Input Validation:

   • If the monthlyChurn is outside the valid range (less than 0 or greater than or equal to 1), it returns an error message: "Invalid input: must be between 0% and less than 100%".
   • This ensures the function only deals with valid churn rates.

3. Churn Rate Conversion:

   • The formula 1 - (1 - monthlyChurn)^12 is used to compute the annual churn rate.
   • It is based on the compounding effect where the annual churn rate is not a simple multiplication of the monthly churn rate but rather reflects cumulative loss over a 12-month period.

4. Output:

   • The computed annualChurn is returned as a decimal value.
   • Optionally, the caller could interpret it as a percentage by further converting it, though the function itself does not explicitly format it as a percentage.

Example Usage:

toAnnual(0.1);  // Input 10% (or 0.1 as decimal). Output will be approximately 0.713 (71.3% annual churn).
toAnnual(10);   // Input 10% as number, normalized first, output will be the same as above (71.3%).
toAnnual(-0.2); // Invalid input, returns an error message.
toAnnual(1);    // Invalid input, returns an error message.

The function is a custom function designed for scenarios such as analytics dashboards (e.g., in spreadsheets or SaaS applications) to help users understand retention and churn trends.