During 2001 (the beginning of the study period), Yahoo! was the most popular US search engine, with a 49% market share; however, in 2003 (the end of the study period), Google surpassed Yahoo! as the leading US search engine [22]. Yahoo! remains a widely used search engine; more than 45 million US Internet users conducted Yahoo! searches in February 2004 [14].

Yahoo! maintains a database of search activity called the Yahoo! Buzz Index [23]. This index provides a search activity score for individual search terms—the words or characters that users type into the Yahoo! search box. Each point of a Yahoo! Buzz Index score equals 0.001% of users searching Yahoo! during the time period of interest (day, week, or month). For example, if 250 out of a total of 1 million users searched for “breast cancer” on January 1, 2001, the Yahoo! “breast cancer” search activity score on this day would be 25 (250/1 million × 100000). For a search term to register a search activity score, it must generate at least 50 searches during the time period for which the score is calculated. Yahoo! search activity generated by search terms can be segmented by country, US state, or selected US cities. This study used daily US Yahoo! search activity data from January 1, 2001 (the earliest date for which Yahoo! search activity data are available) to December 31, 2003. We limited our analysis to Yahoo! searches because at the time of this study no other Internet search engine offered a dataset like the Yahoo! Buzz Index, which provides search activity scores adjusted for the size of the population searching for online information.

Yahoo! employs professional “surfers” or content indexers who manually classify Web pages into one of more than 2000 content categories, such as “movies,” “footwear,” “astrology,” or “cancer or neoplasms.” The Yahoo! Buzz Index classifies search terms in the same content category as the first Web page link that a user “clicks” or activates after conducting a search. For instance, if a user entered the search term “colon” and then clicked on a cancer website, “colon” would be classified as a “cancer or neoplasms” search term. If the user clicked on a grammar website, however, “colon” in that instance would be classified as an “education” search term. When a user does not click on a Web page link or when a user clicks on a Web page link that has not been classified, the Yahoo! Buzz Index categorizes the search term using a variety of algorithms that analyze recent content viewed by the user.

To identify commonly used Internet search terms related to specific cancers, we reviewed the search terms classified in the “cancer or neoplasms” category of the Yahoo! Buzz Index, which generated at least 50 searches in any month from January 2001 to December 2003. This search strategy identified 76 unique search terms, of which 23 were included in the analysis (Table 1). The remaining 53 terms were discarded because they did not relate to a specific cancer or included non-English words. Discarded terms included drug names or treatment modalities, such as “chemotherapy” (n = 19); the names of organizations or events, such as “Race for the Cure” (n = 16); general cancer or anatomy terms, such as “oncology” (n = 11); names of celebrities, such as “Gilda Radner” (n = 5); and the carcinogen “DES” (n = 1). Also, “leucemia” (n = 1), the Spanish word for “leukemia,” was discarded because the Yahoo! Buzz Index does not consistently track foreign words, as it excludes search terms that contain non-English characters. For instance, the Yahoo! Buzz Index would not capture a search term with an accent mark, such as “cáncer colorectal” (Spanish for “colorectal cancer”).

The estimated incidence and mortality for 23 cancers during the study period were obtained from annual burden reports published by the American Cancer Society [24-26]. All cancers with at least 7500 new cases in 2001, 2002, or 2003 were included in the analysis (n = 23) whether or not they were associated with Yahoo! search activity.

News reports associated with specific cancer types were identified using the LexisNexis “US News” database, which includes more than 400 national and regional newspapers, such as the Wall Street Journal and the Baltimore Sun, and a variety of newswire services, such as the Associated Press and UPI (United Press International). News reports published from January 1, 2001, to December 31, 2003, related to specific cancer types were found by locating reports with the identified Yahoo! search activity terms (eg, “breast cancer”) in their headlines. In the case of cancers located in the esophagus and oral cavity, for which no Yahoo! search activity terms were associated, the terms “esophageal cancer” and “oral cancer” were used as the headline search terms.

Descriptive statistics were calculated for the Yahoo! search activity score, estimated incidence, estimated morality, and news coverage volume associated with the cancers included in the study. Spearman rank correlations were used to establish the consistency of these variables across the study period, and the data were aggregated. Next, the relationships between Yahoo! search activity and the potential correlates of interest were tested using Spearman rank correlations.

The relationship between Yahoo! search activity and news coverage was further analyzed for the five cancers with the highest daily Yahoo! search activity. The number of news reports published each day was transformed into a categorical variable with four levels (0 news reports, 1–2 news reports, 3–4 news reports, and 5 or more news reports), and one-way analysis of variance (ANOVA) was used to compare mean daily Yahoo! search activity scores at increasing levels of news coverage. To detect possible periodicity effects, Yahoo! search activity data were visually inspected. Three possible periodicity effects were noted: a rise during weekdays (Monday–Friday) compared with weekends; a rise during national cancer awareness months compared with other months; and a decline during summer months (June–August) compared with other months. These possible effects were tested using t tests. Finally, the Yahoo! search activity associated with several cancers was marked by sharp increases of 100% or more from one day to the next. For these cancers, we identified the three highest peaks in 2003 Yahoo! search activity and investigated corresponding news events.

We tested the relationships between variables by determining Spearman rank correlations between each pair. Statistically significant correlations were found between all variable pairs (Table 2).

The relationship between Yahoo! search activity and its most statistically significant correlate—news coverage—was further analyzed for the five cancers with the highest daily Yahoo! search activity (breast, lung, leukemia, colorectal, and prostate). For these cancers, the number of news reports published each day was transformed into a categorical variable with four levels. The mean daily Yahoo! search activity at each level was compared using ANOVA, and all tests were statistically significant (P ≤ .001). For all five cancers, the mean daily search activity score was higher at each increasing level of news coverage (Table 3).

Three possible periodicity effects were tested: a rise during weekdays (Monday–Friday) compared with weekends; a rise during national cancer awareness months compared with other months; and a decline during summer months (June–August) compared with other months. To test for these potential effects, we used the five cancers with the highest daily mean Yahoo! search activity scores (breast, lung, leukemia, colorectal, and prostate) (Table 4). For all five cancers tested, both mean daily Yahoo! search activity scores and mean daily news reports were higher Monday–Friday than they were Saturday–Sunday (P < .001). Higher mean daily Yahoo! search activity scores were found for breast cancer (P < .001), lung cancer (P < .001), and colorectal cancer (P < .001) during their respective national awareness months. The number of mean daily news reports related to breast cancer (P < .001), colorectal cancer (P < .001), and prostate cancer (P = .007) rose during their respective national awareness months. Mean daily Yahoo! search activity scores for breast cancer (P < .001), lung cancer (P < .001), and leukemia (P < .001) were lower during the summer months than during the rest of the year. While mean daily news reports about breast cancer also decreased during the summer (P < .001), mean daily news reports about prostate cancer rose (P = .01).

On several occasions, Yahoo! search activity scores associated with breast cancer, colon cancer, and prostate cancer were marked by sharp increases of 100% or more from one day to the next. We investigated news events that corresponded with the highest three spikes in 2003 Yahoo! search activity for these cancers. These peaks in “breast cancer” and “colon cancer” search activity all occurred during their respective national awareness months and appeared to be related to news coverage promoting the awareness months. The highest peak in “prostate cancer” search activity (22.34) occurred on July 17 after news reports of a study [27] exploring the association between sexual behavior and prostate cancer risk (Figure 1). These news reports generally focused on the possible protective benefit of masturbation. This study was not covered widely by the US news media, but it generated substantial news coverage in Australia and filtered onto the Internet via chat rooms, message boards, and medical news Web pages. While there was no corresponding spike in “masturbation” search activity, there was a 117% increase in the search activity score (from 61.88 on July 16 to 133.08 on July 17) for “masterbation,” a common misspelling. The second highest spike in “prostate cancer” search activity (14.59) occurred on October 21 after news broke that Academy-Award-winning actor Robert DeNiro had been diagnosed with prostate cancer. This story was widely covered by the US news media, and a 277% increase in “Robert DeNiro” search activity was observed on the same date (from 15.87 on October 20 to 59.90 on October 21). The third highest peak in “prostate cancer” search activity (12.41) occurred on December 29, when a study linking obesity with increased prostate cancer risk [28] was covered by several US news outlets. No corresponding rise in searches for the terms “obesity,” “overweight,” or “weight loss” was observed.

Figure 1

2003 US prostate cancer Yahoo! search activity (each point of a Yahoo! search activity score equals 0.001% of the population searching Yahoo! on any day)