Incidence Rate: An example with Down Syndrome

How common is a particular disease or condition? It depends on what you mean by common. Do you mean how many people have a condition, or do you mean how many new cases of it are there each year?

In other words, are you asking about the probability of you HAVING a disease or of you GETTING a disease?

Yes, I mentioned Down syndrome in the title and I am going to use it as an example and one could argue, and I would agree, that Down syndrome is not, strictly speaking, a disease. That relates to a second point, though, which is that incidence and prevalence are terms and techniques that can be applied not just to disease but to chromosomal disorders and even to risk factors, such as smoking. Now, I’m getting ahead of myself which just shows that one should pipe down and pay attention.

INCIDENCE RATE is the rate at which new cases are occurring.

I downloaded a data set of 40,002 births that occurred in U.S. territories. Did you know that the U.S. administers 16 territories around the world? Well, now you do. Bet you can’t name more than four of them. I’m not sure whether this is a good deal for the people of the territories or not, but I am sure they had 40,000 babies.

Finding the incidence rate for Down syndrome was super-duper easy.

I made a birthweight data set, but unlike the last one, I selected an additional variable, ca_down, and then I recoded that variable.

FILENAME  in “C:\Users\you\Downloads\Nat2014us\Nat2014PublicUS.c20150514.r20151022.txt ” ;

LIBNAME  out “C:\Users\me\mydir\” ;
DATA  out.birth2014 ;
INFILE in  ;
INPUT birthyr 9-12 bmonth 13-14 dob_tt 19-22 dob_wk 23 bfacil 32
mager 75-76 mrace6 107 mhisp_r 115 mar_p $ 119 dmar 120
meduc 124 bmi 283-286 cig1_R 262 cig2_R 263 cig3_r 264 dbwt 504-507 bwtr4 511

ca_down $ 552 ;

if ca_down in (“C”,”P”) then down = “Y” ;
else down = “N” ;

You see, the Down syndrome variable at birth is coded as C for Confirmed, N for No and P for Pending. Well, the “pending” means that the medical personnel suspect Down syndrome but they are waiting for the results of a chromosomal analysis and the confirmed means the analysis has confirmed a diagnosis of Down syndrome. Since I presume that most experienced medical personnel recognize Down syndrome, I combined those two categories (which, if you read the codebook, it turns out that the NCHS folks did, too.)

Then I did

PROC FREQ DATA =  out.birth2014 ;

TABLES down ;

And the result was

down Frequency Percent Cumulative
N 39963 99.90 39963 99.90
Y 39 0.10 40002 100.00

The incidence rate is .10 or 1 per 1,000 .

That’s all incidence rate is – total number of new cases / number at risk

The number at risk of Down syndrome would be all babies born, all 40,002 of them. The number new cases was 39. According to the World Health Organization, the incidence of Down syndrome is between 1 in 1,000 and 1 in 1,100 live births.

So, whatever other fallout there may be of living in a U.S. territory, it doesn’t seem to carry with it any greater incidence of Down syndrome births.

When discussing incidence, a condition like Down syndrome is easy to start with because you have it when you are born, you don’t develop it later. That is not the case with every health condition, though. That’s another blog for another day.

Here’s an interesting note: as often occurs, the most complicated part of this analysis was getting the data. After that, it was easy.



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