Television executives seek programs that can run for several seasons, yet 11 of the 50 longest running primetime shows of all time have appeared on the not-for-profit public television channel PBS.

PBS programs are different from shows on for-profit television channels because they are less subject to the uncertainty of the marketplace. For one, they are allowed more time to develop an audience than shows on broadcast networks that are at the whim of ratings. They also receive funding from foundations and individuals who believe in the content and mission of public service programs.

Many programs on broadcast and public television have an extended life because they are inexpensive. Many also have simple formats, often with a charismatic host, or deal with a timeless topic such as gardening or renovating a house.

To determine the longest running primetime TV shows of all time, 24/7 Wall St. developed a list of primetime television shows using the Internet Movie Database and other sources.

Click here to see the full list of the longest running TV shows of all time.

Six PBS programs are among the 10 longest running shows, and together they have logged a total of 231 years on the air. The science program “Nova” has been on television for 44 years and trails only “60 Minutes” (49 years) in terms of longevity.

Other long-running shows on PBS are “The Victory Garden” (43 years); “This Old House” (39 years); “Nature” (35 years); “Frontline” (35 years); and “Wall Street Week” (35 years). Of those programs, only “Wall Street Week” is no longer on the air.

Among the other so-called legacy networks, 10 of the longest running primetime TV shows are on CBS, eight are on NBC, two on ABC, and three on Fox.

Dr. Robert Thompson, founding director of the Bleier Center for Television and Popular Culture and a trustee professor of television and popular culture at Syracuse University in New York, is an expert on television and its impact on American culture.

Thompson said one reason PBS has the most longest-running primetime TV shows is because the success of its programs is not dictated by market forces.

“Because it is not being directly supported by advertisers, PBS does have the advantage of being able to be more patient with shows,” said Thompson, author or editor of five books on television. “One of the points of the formation of PBS was to allow for programming that might not survive in the ratings marketplace.”

Founded in 1970, PBS is the largest nonprofit, government-funded broadcasting network in the United States.

PBS programs such as “Frontline,” “Nova,” and “American Experience” have not only garnered a loyal audience that has spanned generations, but also critical acclaim.

Thompson also said many PBS shows are in formats with long shelf-lives. “‘Washington Week in Review’ could go on forever,” said Thompson. “‘Nova,’ ‘Frontline,’ ‘Masterpiece Theatre,’ ‘Live from Lincoln Center,’ and others are essentially just umbrella titles for documentaries, performances, and dramas that evolve over time. These formats are much more adaptable to the march of time than a series with a set cast of characters.”

Dr. Amy Franzini, associate professor and chair at Widener University in Chester, Pennsylvania, said programs on public television are given a little more time to build a following than shows on broadcast or cable television. “Today’s newer ‘streaming’ model of viewing — Netflix, Hulu, Amazon — mirrors this a bit,” said Franzini. “Shows have a longer chance to build viewership through word-of-mouth.”

Franzini also pointed out that because PBS is a membership organization, many of the programs have specific sponsors such as foundations or individuals. “Long-lasting funding relationships certainly contribute to their longevity,” Franzini said. “That allows them to cull a longstanding, devoted following.’’

Franzini added that because PBS is a public broadcasting company, it is perceived as more unbiased than corporate-owned entities.

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