Despite “The Big Bang Theory’s” preoccupation with science, it’s become easy to take the show’s higher math — in the form of jaw-dropping ratings, both on CBS and in reruns on TBS and in syndication — for granted.

BigbangsheldonYet while the show was good from the get-go, this week’s Valentine’s Day episode — which was made available in advance — underscores just how deftly the producers have evolved the project, by expanding beyond the four-guys-and-one-girl equation to include a trio of fleshed-out female characters, as well as supporting players beyond that.

The introduction of Mayim Bialik and Melissa Rauch as series regulars has made the show that much richer, and the episode also finds Raj (Kunal Nayyar) taking refuge from being dateless on the holiday with Stuart (Kevin Sussman), the comic-book store proprietor.

Yet if the Stuart-Raj friendship and relationship between Howard (Simon Helberg) and Rauch’s Bernadette have become consistent sources of laughs, the program has also stumbled into something oddly sweet and wonderful in the peculiar interaction between Sheldon (Jim Parsons) and Bialik’s Amy, two scientific geniuses who are both painfully awkward in their interpersonal relationships. At times watching the show in recent weeks, I’ve found myself thinking of “David and Lisa,” a heartbreaking 1962 movie about a romance between two youths in a mental institution.

Bigbang1Admittedly, Sheldon and Amy aren’t that disturbed, but his reluctance to be touched and lack of empathy do make watching them feel around the fringes of what having a “girlfriend” means interesting, as well as funny. (There was also an unexpectedly touching moment in perhaps the season’s funniest episode, where the guys head to a sci-fi convention in “Star Trek” costumes, only to have their car stolen.)

Finally, the producers have managed to sustain the latest Leonard-Penny pairing without hitting any conspicuous snags — no small feat, given the number of shows that have stumbled when it comes to hooking up two primary characters. (For a separate take on the show’s romantic pairings, see this post by my colleague Jon Weisman.)

As for CBS, the network’s sales department has to wake up all smiles every Friday, with “Big Bang” having proved formidable enough to not only beat back “American Idol” but keep “Two and a Half Men” ratings solid, even as the show has drifted creatively.

So analyzing all the available evidence, “Big Bang’s” numbers aren’t just a byproduct of people finding the series in syndication and hopping aboard, although that’s clearly been a factor. More important is that in its sixth season, the program has grown progressively better and more well rounded. And you needn’t be a math wiz to recognize why that winning formula is adding up.