What is a Labor Rate Index
The labour rate index is actually synonymous with a more popular term, the labour cost index.

In the same way that the consumer price index measures inflation of consumer prices, the labour cost index measures the cost of purchasing labour. For example, 12 dollars could buy you more labour 30 years ago than it can now. Inflation indexes like these can be used to normalize historical data. That way we can compare changes and trends across years without worrying about labour cost being inflated.

https://nso.gov.mt/en/nso/Sources_and_Methods/Unit_C2/Labour_Market_Statistics/Pages/Labour-Cost-Index.aspx

https://www150.statcan.gc.ca/n1/en/pub/71-586-x/71-586-x2001001-eng.pdf?st=mrJ1RXhQ