Credit Suisse’s Spencer Wang says that his review of independent audience data shows that Viacom has a real problem on its hands — it isn’t the victim of a possible Nielsen snafu, as Viacom CEO Philippe Dauman has charged. The analyst cut his ad-growth forecast for Viacom’s current quarter in half, to 3%, and shaved 4 cents off his earnings-per-share estimate, to $1.02, after he concluded that “Nick has lost viewership share to the Disney Channel.” Wang doesn’t let Nielsen off the hook completely. He says that Nick’s audience is really down vs last year by mid- to high-single-digit percentages, not the mid-to-high teens that the ratings service reports. What accounts for the drop? Wang doesn’t buy the thesis that Nick viewers continue to watch its shows but on Netflix, which had 140 programs from the  kids’ network in 2011 vs 80 last year and 28 in 2009. Most of this year’s additional Nick content went to Netflix in February, but the ratings drop accelerated in October. He recognizes that the overall audience among children 11 and under is falling — it’s -4% in October and -4.8% in November. Still, ratings at the Disney Channel grew about 10% in both months. That means “some shift in viewer share within the children’s demo” accounts for Nick’s losses. But this isn’t just a blame game. Viacom’s the loser because Nielsen ratings still determine ad rates. And based on the ratings company’s data, Nick is delivering about 1.3M viewers, not the 1.6M Wang estimates it promised advertisers in upfront market sales. That’s significant. A 17% drop in Nick’s ratings this quarter would translate into a shortfall of about 3,000 spots, which he says is about 14.3% of its total inventory. After factoring in the higher prices the network is charging in the scatter market, Wang says Nick’s ad revenues are probably down 9.3% this quarter — not up 8%, as he had predicted. As for 2012, Wang says that he is “giving Viacom the benefit of the doubt that an influx of new original programming on Nick witll help address ratings.” Dauman told analysts this week that he’s “frustrated” by the situation but “next quarter we expect to see stronger ad sales growth because we won’t have that issue” with Nickelodeon’s ratings.