I agree with Emily--this one was a tough assignment. Happy to hear some discussions about my examples, because I'm not 100% confident in my answers.
Assignment:
Defining the time dimension is a fundamental challenge in longitudinal data analysis. The most common choice is age or time since study enrollment, however, for many questions other time dimensions are relevant, for example grade in school, or time before or after stroke incidence, or time since release from prison. Identify an article in the applied literature using a longitudinal analysis based on age as the time dimension, time since study enrollment as the time dimension, and one other possible time dimension (ie, not age or time since study enrollment). For each study, briefly describe the research question, the study sample, the longitudinal design, and the analysis approach. Please post links to the studies.
- Age at menarche
Article: Age at menarche, total mortality and mortality from ischaemic heart disease and stroke: the Adventist Health Study, 1976–88
Article link: https://academic.oup.com/ije/article/38/1/245/698081/Age-at-menarche-total-mortality-and-mortality-from
Research question: Does earlier age at menarche predict mortality, particularly death from ischaemic heart disease and stroke?
Study sample: Female participants of the Californian Seventh-Day Adventist Cohort followed from 1976 to 1988.
Longitudinal study design: A total of 19,462 women were followed throughout the course of their life. Age at menarche was self-reported.
Analysis approach: Relationship between age at menarche and mortality were assessed using Cox proportional hazards regression using attained age (I’m guessing this is age from menarche to end of follow-up?) as the time variable. Models adjusted for possible confounders and an interaction between age at menopause and attained age.
- Time since study enrollment:
Article: Effect of paracetamol on parasite clearance time in Plasmodium falciparum malaria.
Article link: http://www.sciencedirect.com/science/article/pii/S0140673697022551
Significance: Routine antipyretic therapy in children with infectious diseases has long been the source of controversy. Each year, in addition to antimalarial medication, millions of children with Plasmodium falciparum malaria receive paracetamol to reduce fever. However, the usefulness of this practice has not been proven.
Study sample: Children between the ages of 2-7 with asexual P. falciparum parasite loads between 25,000 and 20, 0000 parasites/μL blood treated with intravenous quinine at the Albert Schweitzer Hospital in Lambaréné, Gabon between February and June 1996.
Longitudinal study design: Randomized control trial of 50 children with P. falciparum malaria treated with intravenous quinine, and randomized to receive either mechanical antipyresis alone, or in combination with paracetamol. Rectal body temperature and parasitemia were recorded every 6 hours for 4 days upon study enrollment. Primary endpoints were parasite and fever clearance time.
Dimension of time: Authors used time since enrollment (i.e. time since treatment/admission) to measure the primary outcomes.
Analysis approach: Differences between the groups were assessed by the Mann-Whitney test and χ2 test. Differences between time points within a group were assessed by the Wilcoxon signed-rank test.
3. Trimester when exposed to Dutch Famine (from November 1944 to May 1945)
Article: Decreased birthweights in infants after maternal in utero exposure to the Dutch famine of 1944-1945.
Article link: http://onlinelibrary.wiley.com/doi/10.1111/j.1365-3016.1992.tb00764.x/epdf
Research question: Did in utero exposure to Dutch famine cause decreases in birthweight?
Significance: During the last months of WWII from October 1944 until the surrender of the German forces on May 7, 1945, the Western Netherlands was affected by an acute famine. Circumstances of the Dutch famine provides a unique opportunity to study the long-term effects of this environmental exposure.
Study sample: 1808 first-born singleton offspring of mothers born between January 1, 1944 to June 20, 1946 living in The Netherlands.Mothers were variably exposed to Dutch Famine (first, second, third trimester, or not exposed).
Longitudinal study: Maternal records from the University of Amsterdam teaching hospitals were extracted for births occurring between 1960-1984. Exposure was measured as birth cohorts that represented the trimester the mother was first exposed to the Dutch Famine. Outcomes assessed were birthweight (g), crown-heel length, Quetelet index (kg/m2) and head circumference.
Analysis approach: Differences in birth outcomes were compared with ANOVA. Multivariate linear regression was used to assess differences in offspring birthweights between maternal birth cohorts, testing interactions between place of birth (rural where food supply was more abundant) and maternal birth cohort.