1. Journal Club Notes

2. 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

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

4. 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

5. 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!

6. 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: 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

8. New York Congenital Malformations Registry

9. 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:

10. New York Congenital Malformations Registry

12. https://apps. health. ny

16. 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