Critics are supposed to rend teeth and gnash clothing -- or is it the other way around? -- when a show like "Full House" catches on, but why? The new ABC sitcom seems to follow at least one basic, if often ignored, rule of television: First, do no harm.

A little too loud, a touch too anxious, "Full House," which gets a "preview" at 8:30 tonight on Channel 7 (before settling into its 8 p.m. Friday time slot later this week), nonetheless demonstrates plenty of marksmanship in the old aim-to-please department. It has a nice convivial camaraderie going for it, and one fail-safe ingredient in reserve: the proverbial cute baby.

Bob Saget, a "CBS Morning Program" escapee, and the personable John Stamos, getting more comedically accomplished with each passing series, costar as a young sportscaster and his boppy musician brother-in-law, who join forces to raise the sportscaster's three little girls three months after his wife has died.

As further help, and as laugh insurance, they also enlist David Coulier as pal Joey, a young comic who emulates, though not too convincingly, the style of Robin Williams.

Jodie Sweetin is spunkily adorable as daughter Stephanie, and Candace Cameron is a born crowd-pleaser as little D.J., whose attempt to escape from her bedroom by inching along the top of the draperies is a slapsticky highlight of the program. So is a long sequence in which Stamos and Coulier attempt the very daunting task of diapering the baby.

Yes, we put a man on the moon, but a man still can't put a Pamper on a toddler.

Executive producer Jeff Franklin wrote the script, directed with spirit by Joel Zwick. Tender Moment is cued near the fade-out; tears are dried and wounds healed, and Saget says, "We're still a family. We need to stick together now more than ever." Earlier, someone says, "Everything is going to work out supergreat," and for "Full House," the prognosis may be almost that positive.