Assumption 1: Number of Informal Science Institutions in the United States = 2,500

There is, unfortunately, no accepted, authoritative estimate for the number of informal science institutions in the United States. So we used two main sources for making an estimate of this number: the results of the American Association of Museum’s 2003 survey and the results of our CILS survey.

Based on their 2003 survey results, AAM estimated that there were 2,320 aquaria, arboreta, botanical gardens, children’s museums, nature centers, and science centers within the United States, with an additional 672 natural history / anthropology museums of which only some would come under our definition of an informal science institution (“2003 Museum Financial Information,” American Association of Museums: Washington, DC). The AAM numbers also did not include planetaria or other specialized, science-related institutions. Therefore, it is reasonable to assume that the number of ISIs in the United States is greater than 2,320.

For our current CILS study, we mailed our survey to 2,597 informal institutions that we had tentatively identified as being involved with science. Out of the 514 responding institutions, 475 (92.4%) fell within our definition of “informal science institution” (they had a permanent, physical location and they were a) an aquarium, botanical garden, natural history museum, nature center, planetarium, science center, or zoo; or b) a children’s museum or "other" type of institution that indicated they had “any type of collections, exhibits, or programs that focus on any aspect of science, broadly defined”). If we assume the same percentage of 92.4% ISIs within our overall mailing population, this means there were 2,400 informal science institutions on our mailing list. Since our sources for the mailing list (professional associations and the AAM Museum Directory) are known to not include all institutions, especially smaller ones, the true number of ISIs in the United States is most likely somewhat greater than 2,400.

Based on this, a reasonable ballpark estimate of the current number of ISIs in the United States is 2,600, which is the number we used for the calculations in Table 1. However, for the purpose of making estimates about the overall impact of ISIs, we decided to use a slightly lower, more conservative estimate of 2,500 institutions.

Assumption 2: Number of ISIs with programs for K–12 science education beyond one-day field trips = 1,825 (73% of 2,500)

Within the group of ISIs who responded to our survey, 73% had programs for K–12 science education beyond one-day field trips. If this same percentage holds true for all of the estimated 2,500 informal science institutions in the U.S., then the overall number with programs is 1,825.

Assumption 3: Number of schools served by ISIs with K–12 science programs is 73,000 (40 x 1,825)

This is based on the finding within our survey that the median number of schools served by an ISI is 40. We used the median of 40 rather than the average of 169 because the distribution was highly skewed and we wished to use the more conservative median value.

Assumption 4: There are 118,042 schools (public and private) within the United States

This estimate comes from the National Center for Education Statistics, based on data gathered between 1997 and 2001. From this, we derive the estimate that ISIs serve 62 percent of the schools in the United States (62 percent of 118,042 = 73,000).

Assumption 5: By serving 73,000 schools, ISIs directly or indirectly effect 9,000 districts, 2 million teachers, and 36 million students.

The basis for these estimates comes from the National Center for Education Statistics, using data gathered between 1997 and 2001 concerning the ratio of public to private schools in the United States, the average number of schools per district, the average number of teachers per school, and the average number of students per school.