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.
- Time dimension: age (days of life)
Safety and efficacy of early amino acids in preterm <28 weeks gestation: prospective observational comparison.
Research question: Kotsopoulos et al evaluated the safety and efficacy of early administration of amino acids among preterm infants < 28 weeks gestational age.
Study Sample: the study sample included two groups of preterm infants (each of sample size 54), born at Mount Sinai Hospital in Toronto who were admitted to the Neonatal Intensive Care Unite. One group of children received amino acids parenterally (via intravenous catheter) using standard practice (at about 12-30 hours after birth), and the second group was recruited a year later, after practice changed, and amino acids were administered < 6 hours after birth.
Longitudinal Design: Prospective, before-after (standard change in practice) comparative study. Children were followed over time (days of life).
Analysis Approach: X2-test comparing mean differences of endpoints; t-test or Wilcoxon sum test depending on distribution of continuous variables.
http://www.nature.com/jp/journal/v26/n12/pdf/7211611a.pdf
- Time dimension: time since study enrollment
Cardiovascular Disease Mortality Among Breast Cancer Survivors.
Research question: Bradshaw et al. estimated the relative burden of death due to cardiovascular disease among breast cancer survivors compared to women without breast cancer.
Study Sample: Study included women from the population-based Long Island Breast Cancer Study Project (case-control sample). 1,411 women without breast cancer, and 1,413 breast cancer survivors were included in this study.
Longitudinal Design: women were followed prospectively over time until 2009 (to assess mortality and cause of mortality) since study enrollment in 1996-1997.
Analysis Approach: Hazard ratios were calculated using Cox regression for overall mortality, and a competing risks analysis for cardiovascular disease-specific mortality; sub-distribution hazard ratios for cardiovascular disease mortality was also estimated using the Fine-Gray model.
- Time dimension: calendar month
Beverage purchases from stores in Mexico under the excise tax on sugar sweetened beverages: observational study.
Research question: Colchero et al. investigated the effect of purchases of beverages from stores in Mexico after the sugar-sweetened beverage tax was implemented in 2014.
Study Sample: Sample size was 6,253 households which participated in Nielsen Mexico’s Consumer Panel Services between January 2012 and December 2014.
Longitudinal Design: Purchasing data was obtained from the 6,253 households and provided 205,112 household months between January 2012 and December 2014 (tax started January 1, 2014).
Analysis Approach: Authors used a difference in difference fixed effects model, comparing predicted volumes of mL/capita/day of different beverages to the counterfactual consumption, using pretax trends, had the tax not passed.