Model validation and performance

The concordance correlation for the selected model for male total cigarette smoking prevalence was 0.78, 0.83, and 0.87 at sample sizes 10, 50, and 100, respectively, compared to 0.90 when all data were included (i.e., “in sample”). For women, the corresponding figures are 0.78, 0.85, 0.88, and 0.91. The root mean squared error for the selected model for male cigarette smoking prevalence was 2.7, 2.5, 2.2, and 1.9 for sample sizes 10, 50, 100, and in sample, respectively, while for women the root mean squared error was almost identical at 2.8, 2.4, 2.2, and 1.9 for the same sample sizes. Performance of the selected model for male and female daily cigarette smoking was similar to that for total cigarette smoking.

Bias correction for wireless-only households

We compared model predictions for 2011 that incorporated respondents who could only be reached by cell phone with model predictions that did not incorporate these respondents in order to derive a correction for earlier years in which the wireless-only population was excluded. In 2011, the median difference in total cigarette smoking prevalence between modeled estimates with and without wireless-only respondents included was 1.21 percentage points for men and 1.55 percentage points for women. In 2010, the last year without cell phones, where bias due to their exclusion is expected to be greatest, we corrected 57.3% and 75.4% of counties for males and females, respectively, upward by at least one percentage point and 4.7% and 16.3% of counties for males and females, respectively, upward by at least two percentage points for total cigarette smoking prevalence.

National total cigarette smoking prevalence

Figure 1 shows the national estimates for age-standardized total cigarette smoking prevalence derived from our models. Total cigarette smoking prevalence has declined for males by 1.3% (95% uncertainty interval: 1.2%-1.4%) per year, from 27.3% (26.9%-27.7%) to 22.2% (21.9%-22.5%), and for females by 1.4% (1.2%-1.5%) per year, from 22.2% (21.9%-22.6%) to 17.9% (17.7%-18.2%). Most of this decline took place from 2002 onwards; trends from 1996 to 2002 are relatively flat.

Figure 1 National age-standardized total cigarette smoking prevalence, 1996-2012. Full size image

For comparison, direct (nonmodeled) estimates from the National Health Interview Survey (NHIS) [34], a nationally representative household survey, are also plotted. These estimates have been reweighted to account for the distribution of the population by race, marital status, and educational attainment and then age-standardized; this is for consistency with the modeled BRFSS estimates. Estimates from the NHIS for total cigarette smoking confirm the declines observed in the modeled estimates based on BRFSS data. Further, while estimates from NHIS vary noticeably from year to year, on the whole the level of total cigarette smoking suggested by the NHIS is consistent with that from our models based on BRFSS data.

County-level total cigarette smoking prevalence

Figures 2 and 3 show the age-standardized total cigarette smoking prevalence for males and females, respectively, in 1996 and 2012. (Estimates for the top and bottom 10 counties in 2012 are presented in Tables 1 and 2 for males and females, respectively, and estimates for all counties in all years are presented in Additional file 3.) For males, regions with high levels of cigarette smoking are observed in the South and parts of the Midwest, particularly around Kentucky. High levels are also observed in parts of Alaska, South Dakota, Nevada, and Arizona. Regions of noticeably low cigarette smoking among males are observed in Utah, Colorado, Wyoming, California, Washington, and parts of New England. For females, the highest levels of cigarette smoking are concentrated in Kentucky, West Virginia, Tennessee, Missouri, Oklahoma, Arkansas, and Louisiana; this pattern is somewhat different from the pattern among males where a much larger portion of the South experienced elevated cigarette smoking rates. Higher levels for females are also observed in parts of Alaska, Nevada, Arizona, North Dakota, and South Dakota, while the lowest levels are seen in Utah, Colorado, Wyoming, California, and along the Mexico-Texas border.

Figure 2 Age-standardized total cigarette smoking prevalence, males, 1996 and 2012. Full size image

Figure 3 Age-standardized total cigarette smoking prevalence, females, 1996 and 2012. Full size image

Table 1 Top- and bottom-ranked counties for male total cigarette smoking prevalence, 2012 Full size table

Table 2 Top- and bottom-ranked counties for female total cigarette smoking prevalence, 2012 Full size table

In 1996, the lowest total cigarette smoking prevalence for males was observed in Utah County, UT (15.5% [13.2%-17.6%]), while the highest was found in Northwest Arctic Borough, AK (42.6% [37.0%-48.5%]), a difference of 27.1 percentage points. In 2012, Falls Church City, VA had the lowest prevalence at 9.9% (8.1%-12.0%), while the highest prevalence was still found in Northwest Arctic Borough, AK at 41.5% (35.9%-46.8%), a 31.7 percentage point difference. For females, the lowest prevalence in 1996 was found in Utah County, UT at 9.0% (7.2%-10.8%), which is 27.8 percentage points lower than the highest-observed prevalence that year in Perry County, KY at 36.8% (31.7%-42.1%). In 2012, female cigarette smoking prevalence was still lowest in Utah County, UT (5.8% [4.9%-6.8%]), which was 35.1 percentage points lower than the highest prevalence in that year, in Northwest Arctic Borough, AK (40.8% [34.8%-46.8%]).

Even within a single state there is often substantial variation among counties. The median gap between highest and lowest cigarette smoking prevalence among counties within the same state in 2012 was 14.7 percentage points for males and 13.6 percentage points for females. The largest gap for males in 2012 was observed in Virginia, where there was a 23.6 percentage point gap in cigarette smoking prevalence for men between Sussex County (33.5% [28.6%-38.7%]) and Falls Church City (9.9% [8.1%-12.0%]). For females, the largest gap in 2012 was observed in Alaska, where there was a 25.4 percentage point gap for women between Northwest Arctic Borough (40.8% [34.8%-46.8%]) and Haines Borough (15.4% [12.3%-18.8%]).

In the vast majority of counties, males smoked cigarettes at higher rates than females (Figure 4): in 99.0% of counties in 1996 males had a higher cigarette smoking prevalence than females, while in 2012 the same was true in 96.4% of counties. The gap between male and female total cigarette smoking prevalence has changed with time, however: the median difference between male and female cigarette smoking was 5.4 percentage points in 1996 compared to 3.4 in 2012. Across all counties in 1996, the gap between male and female cigarette smoking ranged from -3.2 percentage points in Colonial Heights City, VA, to 15.3 percentage points in Jefferson County, MS. In 2012, the gap between male and female cigarette smoking ranged from -5.7 percentage points in Menominee County, WI, to 16.5 percentage points in Sunflower County, MS. The correlation between male and female cigarette smoking prevalence was 0.75 in 1996 and 0.81 in 2012.

Figure 4 Difference between male and female age-standardized total cigarette smoking prevalence, 1996 and 2012. Full size image

Figures 5 and 6 show the change in age-standardized total cigarette smoking prevalence from 1996 to 2012, expressed in terms of the annualized rate of change; Tables 3 and 4 give the top and bottom 10 counties in terms of annualized rates of change. Amongst all counties, the median annualized rate of change was -0.9% for males and -0.6% for females. The greatest decline for males was 4.5% (2.6%-6.4%) per year in Falls Church City, VA, while the greatest for females was 4.1% (1.9%-6.4%) per year in Maverick County, TX. The largest increase for males was 1.1% (-0.2%-2.4%) per year in Issaquena County, MS, while the greatest increase for females was 1.7% (-0.4%-3.6%) per year in McMullen County, TX. Only 39.8% of counties for males and 16.2% of counties for females experienced statistically significant declines in cigarette smoking prevalence between 1996 and 2012, though an additional 57.3% of counties for men and 66.1% of counties for women experienced nonstatistically significant declines over this same period. Counties with statistically significant declines represent a disproportionate share of the population, however, such that 74.4% of the adult male population and 61.1% of the adult female population in 2012 lived in counties where the decline in total cigarette smoking prevalence was statistically significant. There were statistically significant increases in only one county for males and in only three counties for females. The correlation between male and female annualized rates of decline in the same county was moderate at 0.55. In most counties, males and females saw cigarette smoking prevalence move in the same direction (Figure 6), however, in 16.1% of counties males experienced declines while females experienced increases and, conversely, in 1.4% of counties females experienced declines while males experienced increases. For both males and females, the correlation between the level of cigarette smoking prevalence in 1996 and the rate of decline between 1996 and 2012 was low: 0.26 for males and 0.15 for females.

Figure 5 Annualized rate of change in age-standardized total cigarette smoking prevalence, 1996-2012. Full size image

Figure 6 Annualized rate of change in age-standardized total cigarette smoking prevalence, females compared to males, 1996-2012. Full size image

Table 3 Top- and bottom-ranked counties for annualized rates of change in male total cigarette smoking prevalence, 1996-2012 Full size table

Table 4 Top- and bottom-ranked counties for annualized rates of change in female total cigarette smoking prevalence, 1996-2012 Full size table

Total cigarette smoking prevalence as well as changes in total cigarette smoking prevalence varied between counties with different mean income levels [35]. Table 5 shows the median total cigarette smoking prevalence in 1996 and 2012 and the annualized rate of decline in total cigarette smoking prevalence over this period among counties in each income quintile (defined in terms of income in 1996). In both 1996 and 2012, the median cigarette smoking prevalence decreased as mean income in 1996 increased. Moreover, the median rate of change between 1996 and 2012 was more negative for higher income quintiles than for lower income quintiles. As a consequence, more counties in higher income quintiles experienced statistically significant declines from 1996 to 2012: for males only 14.1% of counties in the bottom income quintile experienced statistically significant declines compared to 75.4% of counties in the top income quintile; for females only 4.2% of counties in the bottom income quintile experienced statistically significant declines compared to 45.2% of counties in the top income quintile.

Table 5 Total cigarette smoking prevalence and annualized rates of change by income quintile, 1996-2012 Full size table

County-level daily cigarette smoking prevalence

Daily cigarette smoking prevalence is given for all counties in Additional file 4. When we examined the correlation between total and daily cigarette smoking it was very high: 0.95 across both sexes and all years combined. By definition, daily cigarette smoking is always less than total cigarette smoking prevalence, but the median difference between total and daily cigarette smoking among counties increased from 4.3 to 6.6 percentage points in males and 3.6 to 5.4 percentage points in females from 1996 to 2012. This was due to the fact that daily cigarette smoking prevalence has declined faster than total cigarette smoking prevalence: the median annualized rate of decline for daily cigarette smoking prevalence was 1.9% per year for males and 1.4% per year for females, compared to 0.9% per year and 0.6% per year for total cigarette smoking prevalence for males and females, respectively. Rates of decline in total and daily cigarette smoking over the period from 1996 to 2012 are highly correlated, however: 0.93 for both males and females. In 2012 the gap between daily and total cigarette smoking prevalence ranged from 2.7 (Utah County, UT) to 15.3 (Wade Hampton Census Area, AK) percentage points for males and from 1.4 (Utah County, UT) to 11.8 (Wade Hampton Census Area, AK) percentage points for females.