Arthur Shapiro wrote:

What is likely to have failed to cause such an early failure on these bulbs?
I assume the ballast, but would like an expert to confirm. I'm interested in
purchasing electronic replacements, while realizing they are more expensive
than traditional magnetic ballasts, but an intimidated by unfamiliar concepts
such as "ballast factor".

While you're at it, you might consider switching to T8 bulbs. You'll
need to replace the bulb holders as well, but you'll get electronic
ballasts automatically.

Chris