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Welcome to our Ratings Survival Guide, a handy new feature which takes each new fall series on the broadcast networks and tells you the numbers it will need to survive for a second season. In general, a show's success isn't so much where it lands on the weekly ratings chart but rather how it performs in two key categories:

1. How much of its lead-in audience it retains. If a show averages 10 million viewers that's a good thing. But if the show that airs before it is averaging 30 million viewers, that's not a good thing. Why? Because 20 of those 30 million viewers decided to turn off that particular channel when the next show came on.

2. How it performs year-to-year in its time period. A new show is usually in its time period because the network thinks it can do better than it did in that slot last year. In other words, do worse than the show that was canceled after three weeks last fall then you too probably are not long for this world.

As you'll see, together these measurements paint a clear picture on how a particular show is doing. So without further ado, let's begin.

NIKITA (The CW)
(Thursdays at 9:00/8:00c)

With the success of "The Vampire Diaries" last season, The CW is undoubtedly looking for a little more from the Thursday, 9:00/8:00c hour this fall. It's up to "Nikita" then to hopefully replicate the numbers of last year's time period holder "Supernatural." On average, the veteran drama held onto 71.18% of "Vampire's" audience, a solid but not stellar showing. Said numbers however marked a decline from the 2008-09 season, when "Supernatural" held 84.24% of "Smallville's" audience. "Nikita" then has a decent range to play with: 70% or so will keep it going, 75-80% would be considered respectable growth from last season.

A caveat here though: while ABC, CBS, FOX and NBC's target demographic is adults 18-49, The CW's is actually women 18-34. Since that data isn't readily available, we're left with adults 18-49 as the closet approximation. With that in mind, here's how The CW has fared on Thursday nights the past two seasons (viewers are in millions, adults 18-49 is the percentage of said viewers in the demographic; all are based on fast national data, using only first-run episodes):

thursdays in 2008-2010	time	viewers (retention)	adults 18-49 (retention)
The Vampire Diaries: Season 1	8:00 PM	3.67 (N/A)	1.70 (N/A)
Smallville: Season 8	8:00 PM	3.89 (N/A)	1.65 (N/A)
Supernatural: Season 4	9:00 PM	3.19 (82.01%)	1.39 (84.24%)
Supernatural: Season 5	9:00 PM	2.67 (72.75%)	1.21 (71.18%)

COMING WEDNESDAY: NBC's "Outlaw."