Some words used commonly in academic English are Latin (formula, index, matrix, appendix, corpus, etc.), and these words are often pluralized differently than an English word would be. Using the correct Latin plural form (formulae, indices, matrices, appendices, corpora) gives a text a quite formal feel. However, in some fields, Latin words are pluralized as if they were English (formulas, indexes).
All the indexes used for the productivity growth measure have been gathered from the National Accounts at SCB.
Many common Latin words (like stadium) are rarely pluralized as if they were Latin (stadiums is much more frequently used than stadia).