# interpreting my own data (negative confidence interval in epidemiology)

Im trying to measure the absolute change between two means of systolic bloodpressure, and used a simple method for calculating the confidence interval between to means.

My goal is to estimate the absolute change for the TREND between 1980-2000. So to accomplish this im comparing the first and last year (i.e 1980 and 2000, which i already have an point estimate and 95%Cl for, achieved with proc STDRATE in SAS 9.4, using a backgroundpopulation of holland).

Now to the problem ->

For the year 1980 i have a point estimate with 95%cl for systolic bloodpressure at 94.9(73.1 to 120.8)

For the year 2000 i have a point estimate with 95%cl for systolic bloodpressure at 80.3(70.1 to 96.8)

So the absolute change is -14,6, this means that the asbolute decline is 14.6 with the calculated confidence interval of +/- 22,7 (normal distribution).

### Here's the thing-> i cant make any sense of how to interpret an absolute decline of 14.6 (-8.1 to 37.3).

Since im trying to describe trends and already know that systolic hypertension have a decline with 14,6 estimates then how can i describe a lower limit of minus (-)8.1 if its already a decrease.

Hope that this makes any sense, its probably a more epidemiological question then a mathematical or programming one.

In desperate need of help :)!