Table of Contents

Confidence Intervals
Significance of Time Trends
Kessner Index for Prenatal Care
Kotelchuck Index for Prenatal Care

CONFIDENCE INTERVALS

For rates based on simple proportions, VistaPHw uses the formula derived by Fleiss for this calculation (Fleiss, Joseph L.  Statistical Methods for Rates and Proportions.  New York:  John Wiley & Sons, 1981.  p 14.)

Confidence intervals for age-adjusted rates are calculated according to Chiang (Chiang, CL. Standard Error of the Age-Adjusted Death Rate.  Vital Statistics Special Reports, 47(9):275-285, 1961).

Confidence intervals for life expectancies are based on Chiang (Chiang, CL.  Introduction to Stochastic Processes in Biostatistics.  New York:   John Wiley & Sons, 1968).

SIGNIFICANCE OF TIME TRENDS

To assess whether a significant trend over time is present for a given rate, VistaPHw uses the 2 x k chi-square test as described by Armitage (Armitage, P. Statistical Methods in Medical Research, 2e. Boston:  Blackwell Scientific Publications, 1987. p. 372).   If the component of the chi-square value for linearity is greater than 3.84 (corresponding to a less than 5% chance that the trend is due to random variation) then the trend is considered to be significant.

KOTELCHUCK INDEX FOR PRENATAL CARE

To classify a mother's prenatal care history, Kotelchuck first determines the "expected number of visits" based on the gestational age of the infant and the month care began:

Prenatal care is then:

    Inadequate   -  fewer than 50% of expected visits
    Intermediate   -  50 - 79.9% of expected visits
    *Adequate    -   80 - 109.9% of expected visits
   *Adequate±   -  110% or more of expected visits

*combined as "Adequate Care - Kotelchuck" risk factor.

Note 1 -- Expected visits are listed only for care beginning in the first four months of pregnancy.  If care begins after the fourth month, it is classified as "Inadequate" regardless of the number of visits.

Note 2 -- The Kotelchuck index is computed only when the gestational age can be calculated from the date of birth minus the "date last normal menses began" as entered on the birth certificate.  I.e., it is not computed from estimated gestational age.

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