Most of the country’s biggest daily newspaper had lower circulation in the six months from October 1995 through March 1996 than a similar perioda year earlier.
Most of the country’s biggest daily newspapers had lower circulation in the six months from October 1995 through March 1996 than lower circulation in a similar period a year earlier.
So what we do is - compare two periods as part of prepositional phrase...
" in the six months....." Is parallel to " in similar period ...."
https://gmatclub.com/forum/most-of-the-country-s-biggest-daily-newspapers-had-lower-circulation-242378.html
The explanation offered by GMATNinja makes sense.