parallelogram method

[cab691]

Hi, I have a pretty basic question about the parallelogram method.

Let's say there's a single rate change of 8% at 7/1, and we want to put the earned calendar year premium on level. Naively, I'd guess that if seven eighths of the premium comes from policies written before 7/1, we'd use an overall rate factor of

the weighted average. But that's not what the parallelogram method prescribes! Instead, we use the harmonic weighted average:

Why is the harmonic weighted average better than the ordinary weighted average here?

[Colymbosathon ecplecticos]

What are you computing with the parallelogram method?

Spoiler:

On-level premium factors, not average premium. That's why.

[Vorian Atreides]

To add to Coly's post:

Suppose that there's a subsequent +5% rate change in the following year at, say, 4/1.

What factor is needed to bring the year in question's earned premium to on-level?

[cab691]

Yep, figured it out. Thanks a bunch!