Summary statistics
Command:  Statistics Summary statistics 
Description
Allows to calculate summary statistics: mean, median, standard deviation, percentiles, etc.
Required input
In the Summary statistics dialog box you select the variable of interest. You can also enter a filter in the Select field, in order to include only a selected subgroup of cases, as described in the Introduction part of this manual.
You can click the button to obtain a list of variables. In this list you can select a variable by clicking the variable's name.
Options
 Logarithmic transformation: if the data require a logarithmic transformation (e.g. when the data are positively skewed), select the Logarithmic transformation option.
 Test for Normal distribution: see Tests for Normal distribution.
 Click the More options button for additional options:
 Percentiles: allows to select the percentiles of interest.
 Other averages
 Trimmed mean: option to calculate a trimmed mean. You select the percentage of observations that will be trimmed away. For example, when you select 10% then the lowest 5% and highest 5% of observations will be dropped for the calculation of the trimmed mean. See Calculation of Trimmed Mean, SE and confidence interval for computational details.
 Geometric mean. The geometric mean is given by: $$\left ( \prod_{i=1}^n{x_i} \right ) ^\tfrac1n = \sqrt[n]{x_1 x_2 \cdots x_n} = \exp\left[\frac1n\sum_{i=1}^n\ln x_i\right] $$This option is not available when Logarithmic transformation is selected (when Logarithmic transformation is selected, the reported mean already is the Geometric mean).
 Harmonic mean. The harmonic mean is given by: $$\frac{n}{\frac1{x_1} + \frac1{x_2} + \cdots + \frac1{x_n}} = \frac{n}{\sum\limits_{i=1}^n \frac1{x_i}} $$This option is not available when Logarithmic transformation is selected.
 Subgroups: (optionally) select a categorical variable to breakup the data in several (max. 8) subgroups. Summary statistics will be given for all data and for all subgroups.
Results
Variable  WEIGHT 

Filter  TREATMENT="A" 
Sample size  50 

Lowest value  59.0000 
Highest value  105.0000 
Arithmetic mean  77.6800 
95% CI for the Arithmetic mean  74.6535 to 80.7065 
Median  78.0000 
95% CI for the median  71.6034 to 81.3966 
Variance  113.4057 
Standard deviation  10.6492 
Relative standard deviation  0.1371 (13.71%) 
Standard error of the mean  1.5060 
Coefficient of Skewness  0.3216 (P=0.3231) 
Coefficient of Kurtosis  0.2553 (P=0.8118) 
ShapiroWilk test  W=0.9792 
Percentiles 
 95% Confidence interval 

2.5  59.0000 

5  61.0000 

10  64.5000  59.0000 to 67.9795 
25  70.0000  66.0000 to 72.6003 
75  85.0000  80.7999 to 88.7186 
90  94.0000  86.0000 to 99.3810 
95  95.0000 

97.5  99.7500 

BoxandWhisker plot 
Sample size: the number of cases n is the number of numeric entries for the variable that fulfil the filter.
The lowest value and highest value of all observations (range).
Arithmetic mean: the arithmetic mean $\bar{x}$ is the sum of all observations divided by the number of observations n:
95% confidence interval (CI) for the mean: this is a range of values, calculated using the method described later (see Standard Error of the Mean), which contains the population mean with a 95% probability.
Median: when you have n observations, and these are sorted from smaller to larger, then the median is equal to the value with order number (n+1)/2. The median is equal to the 50^{th} percentile. If the distribution of the data is Normal, then the median is equal to the arithmetic mean. The median is not sensitive to extreme values or outliers, and therefore it may be a better measure of central tendency than the arithmetic mean.
95% confidence interval (CI) for the median: this is a range of values that contains the population median with a 95% probability (Campbell & Gardner, 1988). This 95% confidence interval can only be calculated when the sample size is not too small.
Variance: the variance is the mean of the square of the differences of all values with the arithmetic mean. The variance (s^{2}) is calculated using the formula:
Standard deviation: the standard deviation (s or SD) is the square root of the variance, and is a measure of the spread of the data:
When the distribution of the observations is Normal, then it can be assumed that 95% of all observations are located in the interval mean  1.96 SD to mean + 1.96 SD (for other values see table: Values of the Normal distribution).
This interval should not be confused with the smaller 95% confidence interval for the mean. The interval mean  1.96 SD to mean + 1.96 SD represents a descriptive 95% confidence range for the individual observations, whereas the 95% CI for the mean represents a statistical uncertainty of the arithmetic mean.
Relative standard deviation (RSD): this is the standard deviation divided by the mean. If appropriate, this number can be expressed as a percentage by multiplying it by 100 to obtain the coefficient of variation.
Standard error of the mean (SEM): is calculated by dividing the standard deviation by the square root of the sample size.
The SEM is used to calculate confidence intervals for the mean. When the distribution of the observations is Normal, or approximately Normal, then there is 95% confidence that the population mean is located in the interval x̄ ± t SEM, with t taken from the tdistribution with n−1 degrees of freedom and a confidence of 95% (see table Values of the tdistribution). For large sample sizes, t is close to 1.96.
Skewness
The coefficient of Skewness is a measure for the degree of symmetry in the variable distribution. If the corresponding Pvalue is low (P<0.05) then the variable symmetry is significantly different from that of a Normal distribution, which has a coefficient of Skewness equal to 0 (Sheskin, 2011).
Negatively skewed distribution or Skewed to the left Skewness <0  Normal distribution Symmetrical Skewness = 0  Positively skewed distribution or Skewed to the right Skewness > 0 
Kurtosis
The coefficient of Kurtosis is a measure for the degree of tailedness in the variable distribution (Westfall, 2014). If the corresponding Pvalue is low (P<0.05) then the variable tailedness is significantly different from that of a Normal distribution, which has a coefficient of Kurtosis equal to 0 (Sheskin, 2011).
Platykurtic distribution Thinner tails Kurtosis <0  Normal distribution Mesokurtic distribution Kurtosis = 0  Leptokurtic distribution Fatter tails Kurtosis > 0 
Test for Normal distribution: The result of this test is expressed as 'accept Normality' or 'reject Normality', with P value. If P is higher than 0.05, it may be assumed that the data have a Normal distribution and the conclusion 'accept Normality' is displayed.
If the P value is less than 0.05, then the hypothesis that the distribution of the observations in the sample is Normal, should be rejected, and the conclusion 'reject Normality' is displayed. In the latter case, the sample cannot accurately be described by arithmetic mean and standard deviation, and such samples should not be submitted to any parametrical statistical test or procedure, such as e.g. a ttest. To test the possible difference between not Normally distributed samples, the Wilcoxon test can be used, and correlation can be estimated by means of rank correlation.
When the sample size is small, it may not be possible to perform the selected test and an appropriate message will appear. In this case you can visually evaluate the symmetry and peakedness of the distribution using the histogram or cumulative frequency distribution.
Percentiles (or "centiles"): when you have n observations, and these are sorted from smaller to larger, then the pth percentile is equal to the observation with rank number (Lentner, 1982; Schoonjans et al., 2011):
When the rank number R(p) is a whole number, then the percentile coincides with the sample value; if R(p) is a fraction, then the percentile lies between the values with ranks adjacent to R(p) and in this case MedCalc uses interpolation to calculate the percentile.
The formula for R(p) is only valid when
E.g. the 5^{th} and 95^{th} percentiles can only be estimated when n ≥ 20, since
Therefore it makes no sense to quote the 5^{th} and 95^{th} percentiles when the sample size is less than 20. In this case it is advised to quote the 10^{th} and 90^{th} percentiles, at least if the sample size is not less than 10.
The percentiles can be interpreted as follows: p % of the observations lie below the pth percentile, e.g. 10% of the observations lie below the 10^{th} percentile.
The 25^{th} percentile is called the 1^{st} quartile, the 50^{th} percentile is the 2^{nd} quartile (and equals the Median), and the 75^{th} percentile is the 3^{rd} quartile.
The numerical difference between the 25^{th} and 75 percentile is the interquartile range. Within the 2.5^{th} and 97.5^{th} percentiles lie 95% of the values and this range is called the 95% central range. The 90% central range is defined by the 5^{th} and 95^{th} percentiles, and the 10^{th} and 90^{th} percentiles define the 80% central range.
Logarithmic transformation
If the option Logarithmic transformation was selected, the program will display the backtransformed results. The backtransformed mean is named the Geometric mean. Variance, Standard deviation and Standard error of the mean cannot be backtransformed meaningfully and are not reported.
Presentation of results
The description of the data in a publication will include the sample size and arithmetic mean. The standard deviation can be given as an indicator of the variability of the data: the mean was 25.6 mm (SD 3.2 mm). The standard error of the mean can be given to show the precision of the mean: the mean was 25.6 mm (SE 1.6 mm).
When you want to make an inference about the population mean, you can give the mean and the 95% confidence interval of the mean: the mean was 25.6 (95% CI 22.4 to 28.8).
If the distribution of the variable is positively skewed, then a mathematical transformation of the data may be applied to obtain a Normal distribution, e.g. a logarithmic or square root transformation. After calculations you can convert the results back to the original scale. It is then useless to report the backtransformed standard deviation or standard error of the mean. Instead, you can antilog the confidence interval in case a logarithmic transformation was applied, or square the confidence interval if you have applied a square root transformation (Altman et al., 1983). The resulting confidence interval will then not be symmetrical, reflecting the shape of the distribution. If, for example, after logarithmic transformation of the data, the mean is 1.408 and the 95% confidence interval is 1.334 to 1.482, then you will antilog these statistics and report: the mean was 25.6 mm (95% CI 21.6 to 30.3).
If the distribution of the variable is not normal even after logarithmic or other transformation, then it is better to report the median and a percentiles range, e.g. the interquartile range, or the 90% or 95% central range: the median was 25.6 mm (95% central range 19.6 to 33.5 mm). The sample size will be taken into consideration when you decide whether to use the interquartile range or the 90% or 95% central range (see percentiles) (Altman, 1980).
The precision of the reported statistics should correspond to the precision of the original data. The mean and 95% CI can be given to one decimal place more than the raw data, the standard deviation and standard error can be given with one extra decimal (Altman et al., 1983).
Finally, the summary statistics in the text or table may be complemented by a graph (see distribution plots).
Literature
 Altman DG (1980) Statistics and ethics in medical research. VI  Presentation of results. British Medical Journal 281:15421544.
 Altman DG (1991) Practical statistics for medical research. London: Chapman and Hall.
 Altman DG, Gore SM, Gardner MJ, Pocock SJ (1983) Statistical guidelines for contributors to medical journals. British Medical Journal 286:14891493.
 Campbell MJ, Gardner MJ (1988) Calculating confidence intervals for some nonparametric analyses. British Medical Journal 296:14541456.
 Lentner C (ed) (1982) Geigy Scientific Tables, 8^{th} edition, Volume 2. Basle: CibaGeigy Limited.
 Schoonjans F, De Bacquer D, Schmid P (2011) Estimation of population percentiles. Epidemiology 22: 750751.
 Sheskin DJ (2011) Handbook of parametric and nonparametric statistical procedures. 5^{th} ed. Boca Raton: Chapman & Hall /CRC.
 Westfall PH (2014) Kurtosis as Peakedness, 1905  2014. R.I.P. The American Statistician 68:191195.
See also
Recommended book
Essentials of Medical Statistics
Betty Kirkwood, Jonathan Sterne
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