Statistics for clinicians. 5. Interval data (I).

Interval data may be discrete or continuous. They are usually summarized by the average (arithmetic mean). Sometimes, for example when the possible values in a series change by a constant multiple, we need to use the geometric mean. To obtain the overall or mean percentage of a series of percentage values, we need to calculate their weighted mean. The variability of observations in a sample is measured by the standard deviation, and the variability of sample means is measured by the standard error of mean. Confidence interval is a range which contains the population mean with a known probability. It is obtained by deducting from the sample mean, and adding to it, "t" times the SEM, the value of "t" depending on the desired confidence level (1-P) and the sample size (N). The significance of difference between the mean of two sets of unpaired interval data (MA-MB) is tested by Student's t-test. If the data are paired, the significance of the mean difference (MD) is tested by paired t-test. Ordinal data, ie, grades and ranks, may be analyzed by means of the t-test which is more sensitive and allows more refined analyses if needed.

Statistics for clinicians. 3. Nominal data (II)

When the percentage occurrence of an event in a series of groups shows a linear trend, the standard chi-square test may not reveal its significance. The data should then be analyzed by z-test for a linear trend. When the results of several similar experiments show a trend in favor of one group, but the differences in individual studies are not significant, the data from all studies can be analyzed together by the z-test for significance of the trend. In some studies, patients need to be followed up for long periods for the occurrence of an event. Here, we are interested not only in the frequency of the event, but also how soon it occurs or how long it is delayed. For this purpose the data should be analyzed by the life table method (also known as the logrank method or the Mantel-Haenszel method). Diagnostic tests are usually evaluated by their sensitivity and specificity. However, what is also important for a clinician is their predictive value, which depends on the prevalence of the condition.

Statistics for clinicians. 2. Nominal data (I).

Nominal data consist of items assigned to well defined classes. They are presented as a proportion or percentage of the total. From a sample percentage we can estimate the population percentage with a desired degree of confidence, using standard error of the percentage as a measure of chance variation. We can compare the difference in percentage between two groups by means of the z-test or the chi-square test. If the number of observations is less than 40, Fisher's test is preferable to the chi-square test. When chi-square test is done on 2x2 tables, Yates' correction is recommended. For tables larger than a 2x2, Yates' correction is not used. When testing paired data for significance of difference, we need to use McNemar's modification of Chi-square test. Chi-square test is useful not only to test the significance of differences but also to test the significance of an association between attributes.

Statistics for clinicians. 1. Introduction.

Statistics is a way of thinking about variable events. The relative frequency with which an event occurs is called its probability (P). By convention, events with a probability of 5% or less (P less than = 0.05) are considered rare or "significant". Data may be of nominal (categories), ordinal (grades), or interval/ratio type (measurements). Statistical methods are helpful for: summarizing data; making estimates for populations; defining "normal" range; testing association between attributes; measuring correlation between variables; computing one variable in terms of others; and testing the significance of differences between groups.

[Further experience with MCC (Merchant's Copper Coil)]. / Notre experience complementaire du MCC.

PIP: Data is presented on the use of the Merchant's Copper Coil (MCC) IUD by 400 private patients between March 1981-June 1985. The MCC is a polyethylene IUD in the shape of an open ring largely enveloped in a copper thread .2 mm in diameter and with a surface area of 251 sq mm. Overlapping of the extremities of the MCC allows it to adapt to uterine activity and to all types of uterus. Danger of perforation during a vigorous uterine contraction is minimized by the absence of protruding extremities. Insertion is done using a tube of 3.6 mm in diameter, making the procedure easy and painless even in nulliparas. The MCC is available in different sizes. The size selected in the series was not significantly related to the age of the woman but was slightly related to parity. The average age of users was 26.5 years and the average parity was 1.4. All insertions were done in the office without anesthesia except in 1 nullipara who needed a local cervical anesthetic. 46.7% of users were 20-25 years old, 31.3% were 26-30, and 16% were 31-35. 10.7% were nulliparas, 50.3% had 1 child, 31.5% had 2 children, 5.5% had 3, and 2.0% had 4 or more. 83.5% had secondary educations or better. 80.5% had interval insertions and 19.5% had insertions 2-4 weeks after 1st trimester abortions. Insertions were without complications except for spotting or bleeding in 9 cases lasting from a few hours to 3 days. Between March 1981-June 1985, 118 women used the MCC for 2 years and the total number of woman months of use was 6864. In the 2nd year of use, the cumulative net rates using the Tietze method and numbers of events respectively were .3 and 1 for pregnancy, 4.1 and 14 for expulsion, .6 and 2 for removal because of bleeding, 0 and 0 for removal because of pain, 5.0 and 12 for removal because of other medical reasons or to change the IUD, 13.4 and 34 for desired pregnancy, and 2.3 and 6 for personal reasons. The continuation rate was 91.5% after 1 year and 74.3% after 2 years. There were no cases of perforation due to the IUD. The crude rates of termination per 100 users after 1 and 2 years respectively were 0 and .4 for pregnancy, 3.3 and 4.5 for expulsion, .6 and .6 for bleeding or pain, 1.1 and 5.8 for other medical reason, 3.7 and 15.4 to become pregnant, and 1.0 and 2.5 for other personal reason.^ieng