I need the statistic having to do with longevity of brands/brand
names. My client recalls hearing something to the effect that, of the
top 20 brand names in 1950, 19 of them are still in the top 20. I need
that statistic or
something close to it that supports the idea that strong brands have
great longevity.

I believe your client was referring to a well-known study that
appeared in "Advertising Age," in which it was found that, in 19 of
the 25 product categories studied, the leading brand had maintained
its leading status over a 60-year period.

"An interesting study was conducted about the longevity of famous
brands in 1983 by Advertising Age magazine (later by American
Association of Advertising Agencies in 1985). The magazine reported:
'The brand leaders in 1923 in the US and in 1933 in the UK still are
and would continue to be brand leaders to a very large extent. Names
like Kellogg's, Cadbury's, Colgate, Gillette, Kodak, Del Monte,
Wrigley, Coca Cola, Lipton and Goodyear will ring a bell with most
people.'...

It is year 2002 now and there is definitely no argument against what
Advertising Age had reported about twenty years ago. The
above-mentioned names are still the leaders in their category and
there is no doubt that they will continue to remain so in the years
ahead."

ICFAI Press: Global Brands
http://66.102.11.104/search?q=cache:M43iyH9XAhYJ:www.icfaipress.org/books/Global%2520Brands(Case%2520Study%2520Series).htm

"In 1983 the Advertising Age published a study of brand leaders. In 19
of the 25 product categories studied, the brand leader was still the
brand leader after 60 years...

Advertising Age, September 19, 1983 p32 Study: majority of 25 leaders
in 1923 still on top."

Marketmind: For Brand Leaders, Defence is the Best Offence
http://66.102.11.104/search?q=cache:W5o2QEnJfEQJ:www.marketmind.com/publicns/apm9702.html

"Gillette. Coke. Tide. These brands not only dominate their respective
categories but have done so for several decades. The thesis of
enduring market leadership is not one of isolated examples. Many
researchers report that once brands capture leadership they hold that
leadership for decades. One article claims that 19 of 25 market
leaders held the market leadership for at least 60 years!...

"Brand                       1923 Rank   1983 Rank
Swift's Premium Bacon           1           1
Kellogg's Corn Flakes           1           3
Eastman Kodak Cameras           1           1
Del Monte Canned Fruit          1           1
Hershey's Chocolates            1           2
Crisco Shortening               1           2
Carnation Canned Milk           1           1
Wrigley Chewing Gum             1           1
Nabisco Biscuits                1           1
Eveready Flashlight Batteries   1           1
Gold Medal Flour                1           1
LifeSavers Mint Candies         1           1
Sherwin-Williams Paint          1           1
Hammermill Paper                1           1
Prince Albert Pipe Tobacco      1           1
Gillette Razors                 1           1
Singer Sewing Machines          1           1
Manhattan Shirts                1        Top 5
Coca-Cola Soft Drinks           1           1
Campbell's Soup                 1           1
Ivory Soap                      1           1
Lipton Tea                      1           1
Goodyear Tires                  1           1
Palmolive Toilet Soap           1           2
Colgate Toothpaste              1           2

Source: 'Study: Majority of 25 Leaders in 1923 Still on Top,'
Advertising Age, 1983, p. 32."

Will and Vision: Are Pioneers Really Blessed?
http://www.willandvision.com/resources/Will&Vision.pdf

You may be interested in reading this article, which partially refutes
the conclusions of the "Advertising Age" study, claiming that the
samples used in the earlier study were biased:

What History Tells Us About the Endurance of Brands
http://www.stern.nyu.edu/Sternbusiness/fall_winter_2000/brands.html

Search terms used:

"longevity" + "brands"
"longevity" +"brand names"
"endurance" + "brands"
"endurance" + "brand names"
"advertising age" + "study" + "brands"

I hope this is helpful. If anything is unclear, or if a link doesn't
work for you, please request clarification; I'll gladly offer further
assistance before you rate my answer.

Best regards,
pinkfreud

Perfect! Exactly what I was looking for. Thanks.