Journal Club Notes

Learning Objectives Identify characteristics of the Poisson distribution Distinguish between one-tailed and two-tailed tests for statistical significance Identify data sources for chromosomal abnormalities and birth defects ex- New York State and discuss the limitations of these data Report descriptive statistics, based on the research question

Sparks TN, et al. “Observed Rate of Down Syndrome in Twin Pregnancies”, Obstetrics and Gynecology, vol. 128, no. 5, November 2016.

Poisson Distribution Distribution of a discrete random variable Takes on a finite set of values Used for counts with values equal to who numbers 0, 1, etc. Used for counts per unit of time, especially for rare events Incidence of Down Syndrome – number of new cases over time in population at risk Assumes events happen randomly and independently in time at a constant rate Within each age cohort, Down Syndrome occurs randomly and independently in the population Assume monozygotic pregnancies have equivalent risk to singletons Assume dizygotic pregnancies have twice the risk of at least one affected fetus

Two-tailed vs. One-tailed Tests Two-tailed test used when you want to know if there is a difference between the groups in either direction For example, when comparing the mean age of two groups, it will test whether the means are different from one another One-tailed test is used when you want to know if there is a difference between the groups in one direction For example, when comparing mean age of two groups, test whether: The mean of one group is significantly lower than that of the other group The mean of one group is significantly higher than that of the other group Used when you have a hypothesis about the direction of the effect One-tailed test requires smaller sample than two-tailed test to reach significance DO NOT USE JUST TO ACHIEVE STATISTICAL SIGNIFICANCE!

Sources of Data for Birth Defects and Chromosomal Abnormalities New York Statewide Perinatal Data System – Electronic Birth Certificate Completed for each live birth Only includes live born infants with Down Syndrome Excludes terminations, fetal deaths with Down Syndrome Underestimates the rate of Down Syndrome in pregnancies Onondaga County rate for 2015: 21.2 per 10,000 live births Collects information on whether the infant has a congenital anomaly from a list of birth defects and chromosomal abnormalities Whether it was diagnosed prenatally How it was diagnosed – methods need to be updated

New York Congenital Malformations Registry http://www.health.ny.gov/diseases/congenital_malformations/

New York Congenital Malformations Registry Manual for Congenital Malformations Registry Identifies who to report How to code the conditions Chromosomal abnormalities begin on page 62 Available online at: http://www.health.ny.gov/diseases/congenital_malformations/docs/icd.pdf

New York Congenital Malformations Registry

https://apps. health. ny https://apps.health.ny.gov/statistics/environmental/public_health_tracking/tracker/index.html#/bdcounty

Reporting Descriptive Statistics Row versus column percents Table 1 reports the percent of pregnancies by race-ethnicity Total number of singleton and twin pregnancies are in the denominators Each column adds up to 100% Interpreted as: Among singleton pregnancies, what proportion are to which race-ethnicity 26.2% of singleton pregnancies were to white women Alternative is for row to add up to 100% Interpreted as: Among pregnancies of white women, what proportion are singletons and what proportion are twins