Demography: Population dynamics in Zimbabwe (updated 9/4/2008)

  1. The following data for Zimbabwe come from the U.S. Bureau of the Census International Data Base [IDB, http://www.census.gov/population/international/] (accessed 9/4/2008).

	Demographic indicators in Zimbabwe	2000	2025
	Births per 1,000 population	31.4	27.2
	Deaths per 1,000 population	21.6	10.1
	Rate of natural increase (percent)	0.98	1.71
	Annual rate of growth (percent)	0.45	1.71
	Life expectancy at birth (years)	38.9	59.3
	Total fertility rate (per woman)	3.9	3.3

Which one of the measures listed below is the least affected by the population age distribution? (Choose one best answer and provide a brief statement of support.)

A.   Births per 1,000 population
B.   Deaths per 1,000 population
C.   Rate of natural increase (percent)
D.   Annual rate of growth (percent)
E.   Total fertility rate (per woman)

  *2. In the summary table, the rate of natural increase is larger than the annual rate of growth in 2000, but the two measures are identical in the projection for 2025.

  2a. Use data in the table to show the derivation of the rate of natural increase in 2000.

  2b. What does the fact that in 2025 the rate of natural increase is identical to the annual rate of growth imply?

Access the table of age-specific fertility rates for Zimbabwe for 2000 and 2025 in the IDB (http://www.census.gov/population/international/), click "International Data Base", select Zimbabwe, specify 2000 and 2025 as the years (holding down the Ctrl key after the first click makes it possible to select multiple years), click the "Country Results" tab, and select the table "Fertility Rates".

  3. Between 2000 and 2025, the total fertility rate (TFR) is projected to decrease from 3.9 to 3.3. Which 5-year age group is responsible for the largest (at least slightly) portion of this decline, and by how many births per woman does it decline for that age group? Show the answer to 3 significant figures and include a brief explanation.

The following questions relate to the population age-sex structure of Zimbabwe in 2000 and 2025 as projected by the Census Bureau's International Database (see above for link). Click the tab for "Country Results" and select the table "Midyear Population by Age and Sex" to see a table, or click the tab for "Population pyramids" to see graphs and then use the link below the graphs to download a spreadsheet in Excel or CSV format. For your convenience and to avoid confusion if the database should change again, I have placed the graphs and a spreadsheet with the data in the module folder in Sakai. Assume that all population figures are as of July 1.

  4. What is the birth cohort for persons age 20-24 years in the 2000 population pyramid (i.e., in what years were these people born)? Provide a brief statement of support with your answer.

  *5. Which 5-year age group, between 0 and 79 years of age, has the most unbalanced sex ratio in 2025, i.e., the most members of one sex (males or females) for each member of the other sex? Provide a brief explanation or statement of support.

  6. [Not assigned in Spring 2013] Calculate the sex ratio (males:females) for men and women ages 15-49 years old in 2025.

  7. [Not assigned in Spring 2013] Which 5-year cohort (age group) of women between 0 and 39 years of age in 2000 has experienced the highest attrition (as a percentage of baseline) from 2000 to 2025? Give your answer as their age range in 2000, the attrition percentage, and a short explanation.

HIV trends in Zimbabwe: The following questions refer to the article "HIV decline associated with behavior change in Eastern Zimbabwe", by Simon Gregson et al., Science 2006 (February 3). Unless otherwise indicated, please interpret questions in the context of the article. We do not expect you to understand the article completely. Page and column references refer to the Gregson et al. article unless otherwise indicated. The Science magazine website has supporting online material for this article. There was also a commentary in the same issue should you wish to read it.

  8. [Not assigned in Spring 2013] "Surveillance data indicate that HIV prevalence has declined in several countries in east Africa." (p664,c1) Which of the following statements about the dynamics of incidence and prevalence is(are) true, assuming that all other factors remain unchanged? Briefly explain each of your answers.

  8a. Prevention of new HIV infections through increased condom use will tend to reduce HIV prevalence

  8b. Prolongation of HIV survival through more widespread use of antiretrovirals will tend to increase HIV prevalence.

  8c. Outmigration of persons infected with HIV will tend to reduce HIV prevalence.

  8d. Reduced HIV transmission from detection and treatment of other sexually transmitted diseases will tend to increase HIV prevalence.

Data from the baseline and follow-up surveys are presented in Table 2 in the supplementary online material (table S2) and have been extracted into an Excel workbook in the module folder in Sakai. Data from the "closed cohort" are presented in table S4 and have also been extracted into the workbook. Answer the following questions using the data from these tables and from the Gregson et al. article.

  *9. "HIV prevalence in the 12 study sites was observed to decline over an average 3-year intersurvey interval from 23.0% to 20.5% . . ." (p664,c2). How many persons with HIV were detected during the baseline survey? Include the calculation.

10. The numbers of men and women surveyed in 1998-2000 and 2001-2003 were: 4,320 men and 5,134 women at baseline and 3,047 men and 3,972 women at follow-up. The HIV prevalences for each group are given in the article and in table S2. If the HIV prevalences at follow-up had been 22.3% for men and 18.2% for women, instead of the other way around, what would the overall HIV prevalence have been at follow-up? Show the calculation.

11. "The decline in HIV prevalence was most pronounced in men aged 17 to 29 years, from 10.6% to 8.1% (a decline of 23%; P<0.01), and in women aged 15 to 24 years, from 15.9% to 8.0% (49%; P<0.0001). How many fewer HIV-positive men aged 17 to 29 years were surveyed at follow-up than at baseline? Show the calculation. (Suggestion: Do not attempt this calculation for the women.)

12. "The age pattern of change in HIV prevalence is consistent with one that occurs through the natural dynamics of an HIV epidemic (12), but the quantum of the declines recorded in younger age groups, over a relatively short 3-year period, and the concentration of the decline among people with secondary school education (13) strongly suggested a contribution of sexual behavior change (3)." (p664,c2-3). Which of the following comparisons of age-specific prevalences in Table S2 gives the strongest indication for an influence of HIV-related mortality or migration on prevalence rather than (or as well as) behavior change? Choose one best answer and provide a brief explanation or statement of support.

A. 24-26 year-old men at baseline compared to 24-26 year-old men at follow-up
B. 24-26 year-old men at baseline compared to 27-29 year-old men at follow-up
C. 30-32 year-old women at baseline compared to 30-32 year-old women at follow-up
D. 30-32 year-old women at baseline compared to 33-35 year-old women at follow-up

13. [Not assigned in Spring 2013] "Surveillance data from local antenatal clinic attendees indicated modest declines in HIV prevalence overall (21.1% to 19.2%; AOR, 0.87; 95% CI, 0.71 to 1.06) and at young ages (fig. S1)." (p664,c3). What are at least two possible changes in sexual behavior that would lead to a reduction in HIV transmissions in the population but would not, at least initially, lead to a reduction in HIV incidence and prevalence in antenatal clinic attenders? Explain briefly (60 words maximum).

14. "The contributions of mortality and new HIV infections to changes in HIV prevalence observed in the closed cohort are shown in Fig. 2 and table S4." (p664,c3)

  14a. "HIV incidence was highest in men aged 20 to 44 years and women aged 15 to 29 years." Use the data in table S4 (see the spreadsheet in folder for this module) to calculate the incidence rate for women aged 15 to 29 years. Show the calculation (to 3 significant digits) and state the result in one sentence.

  14b. The "relative risk" of death for HIV-infected men was 11.3 (p664,c3). Use the data in table S4 to show the calculation of this "relative risk". Also, state the meaning of this "relative risk" in one sentence.

*15. The HIV incidence rate for 15-17 year-old women was 1.29 per 100 women-years (table S4). Use this incidence rate to estimate the 3-year HIV incidence proportion (cumulative incidence) in a truly closed cohort of 15-17 year-old women who experienced this incidence rate. Show the calculation, and state the result in one sentence.

16. [Not assigned in Spring 2013] The HIV test used at baseline (the ICL Dipstick) "has been evaluated in Zimbabwe and shown to have high sensitivity and specificity (99.6% in each case)" (supplemental online material, p3).

  16a. Suppose that the true number of HIV-infected participants at baseline had been 2,000. How many HIV cases would we expect to have been missed with this test sensitivity? Show the calculation.

  16b. Continuing with the assumption of 2,000 actual cases, and assuming that no confirmatory test had been done, how many false positives would be expected from the baseline survey of 9,454 persons? Show the calculation.

  16c. Use your answers to 16a and 16b to calculate the positive predictive value of the ICL Dipstick if used without a confirmatory test. Show the calculation and provide a one-sentence interpretation of the result.

  16d. If the same test had been employed, without a confirmatory test, in a study population in which the actual HIV prevalence was only 1%, what would the reported prevalence have been? Show the calculation, and give your answer to 3 significant figures.

17. [Not assigned in Spring 2013] The supplemental online material reports that the authors also conducted a community-randomized controlled trial in these communities during the period between the two survey rounds, but there was no evidence for more rapid sexual behavior change nor greater reduction in HIV incidence in the intervention sites. Which of the following statements about randomized trials is(are) true? Provide a brief explanation for each answer.

  17a. According to Aschengrau and Seage, randomization of communities was not a suitable design for testing the effects of medical services and health education.

  17b. Randomization guaranteed that the intervention and control sites were equivalent in this particular study.

  17c. An intention-to-treat analysis preserves the statistical advantages of randomization.

  17d. A crossover design is a good choice for a trial to alter sexual behavior.

*18. "Owing to the long average incubation period of HIV infection, HIV prevalence reflects the accumulation of infections over a period of more than 10 years and is insensitive to behavior change." (p666,c2). Which one of the following statements best explains the meaning of the quoted sentence? (Choose one best answer and provide a brief explanation or statement of support.)

  A. Since HIV develops over a prolonged period following exposure to the virus, many years of behavioral change are required to prevent HIV disease.

  B. Since most HIV infections in a population are longstanding ones, HIV prevalence is slow to reflect reductions in HIV incidence that may result from behavioral change.

  C. Since there is a prolonged period between infection with HIV and the manifestation of symptoms, it can take many years before a population sees the need to make behavioral changes to prevent new infections.

  D. Since HIV cannot be cured, behavioral change can reduce only HIV incidence and not HIV prevalence.