 S 
Sample
A selected subset of a population. A sample may be random or nonrandom and it may be representative or nonrepresentative.
Scatter Diagram
A graph in which each dot represents paired values for two continuous variables, with the xaxis representing one variable and the yaxis representing the other; used to display the relationship between the two variables; also called a scattergram. .
Seasonality
Change in physiological status or in disease occurrence that conforms to a regular seasonal pattern. .
Secondary Attack Rate
A measure of the frequency of new cases of a disease among the contacts of known cases. .
Secular Trend
Changes over a long period of time, generally years or decades. .
Sensitivity
The ability of a system to detect epidemics and other changes in disease occurrence. The proportion of persons with disease who are correctly identified by a screening test or case definition as having disease.
Sentinel Surveillance
A surveillance system in which a prearranged sample of reporting sources agrees to report all cases of one or more notifiable conditions. .
SexSpecific Mortality Rate
A mortality rate among either males or females. .
Skewed
A distribution that is asymmetrical. .
Specificity
The proportion of persons without disease who are correctly identified by a screening test or case definition as not having disease. .
Sporadic
A disease that occurs infrequently and irregularly. .
Spot Map
A map that indicates the location of each case of a rare disease or outbreak by a place that is potentially relevant to the health event being investigated, such as where each case lived or worked. .
Standard Deviation
The most widely used measure of dispersion of a frequency distribution, equal to the positive square root of the variance. .
Standard Error (OF THE MEAN)
The standard deviation of a theoretical distribution of sample means about the true population mean. .
Sufficient Cause
A causal factor or collection of factors whose presence is always followed by the occurrence of the effect (of disease). .
Surveillance
see PUBLIC HEALTH SURVEILLANCE .
Survival Curve
A curve that starts at 100% of the study population and shows the percentage of the population still surviving at successive times for as long as information is available. May be applied not only to survival as such, but also to the persistence of freedom
