Module 3 Basic Statistical Concepts

1
Module 3Basic Statistical Concepts
2
Objectives for Module 3

• By the end of the module, participants should be
  able to
• define basic statistical terms
• identify types of survey errors.

3
The DHS Surveyors Football Team
15
16
16
17
18
19
20

• Mean

• Median

• Mode

4
The DHS Surveyors Football Team
15
16
16
17
18
19
20

• Mean
• Also known as average
• The sum of all values divided by the number of
  values
• (15161617181920) 7 17.3 years

• Median
• The value that divides the sample into two
  separate halves when ordered, the median is the
  middle value.
• In this case, the median is 17.

• Mode
• The value that occurs most frequently. In this
  case, the mode is 16.

5
The DHS Surveyors Football Team
15
16
16
17
18
19
20

• Another look at Median
• What if we add another 20 year old boy to the
  sample?
• Now there is no clear middle value, the middle is
  between 17 and 18.
• To calculate the median, we take the average of
  the 2 middle values.
• So, (17 18) 2 17.5

20
6
Outliers
15
16
16
17
18
19
60

• What if a 60 year old replaces the 20 year old on
  the football team?
• The median is still 17.
• But the mean is now 23.
• Is mean a good summary statistic in this case?
• Which measure (mean or median) would be a better
  way to describe your team?
• The 60 year old is called an outlier a value
  that lies far outside the normal range of your
  data. Outliers have a strong effect on the mean,
  but not the median.

7
Frequency
15
16
16
17
18
19
20
8
Range
15
16
16
17
18
19
20
Range is 15-20
15
16
16
17
18
19
60
Range is 15-60
9
Percentage
15
16
16
17
18
19
20
A percentage is a way of expressing a number as
part of 100. What percentage of boys is
16? Answer (2 boys who are 16 total number
of boys) x 100 (2 7) x 100 28.6
10
Percentages in the DHS

• Uganda 2005 The percentage of women age 15-49
  who have attended secondary school or higher is
  21.3 percent. The numerator is the number of
  women age 15-49 who has attended secondary
  school. The denominator is the total number of
  women age 15-49.
• of women age 15-49 who attended
• secondary school or higher
• All women 15-49

X 100
11
Rates in the DHS

• HIV Prevalence Rate
• All 15-49 year olds who are HIV-positive
• All 15-49 year olds in the country
• Note In a Rate, the Denominator INCLUDES the
  numerator- the denominator is the total of which
  the numerator is a subset.

12
Rates in the DHS

• Rates can be expressed per 1,000, or even
  100,000, when the number of events is small
• Neonatal mortality in Ethiopia in 2005 was 39
  deaths per 1,000 live births.
• For every 1,000 children born, 39 of them died
  within the first month of birth

13
Ratios in the DHS

• A ratio compares two quantities with different
  units. For example, if there are 4 boys and 1
  girl in a class, the ratio of boys to girls is
  41 (read 4 to 1).
• In a ratio, the denominator does NOT include the
  numerator.
• Ratios can also be expressed as a fraction.
• DHS example maternal mortality ratio - the
  number of maternal deaths per 100,000 live births.

14
Rates and Ratios
A.
B.

• Number of maternal deaths
• ______________________
• Number of women giving birth in a country

• Number of maternal deaths
• ______________________
• Total number of female deaths in a country

Module 5, Slide 14
15
Standard deviations
99.7 of population is within 3 SDs of mean
95 of population is within 2 SDs of mean
68 of population is within 1 SD of mean
Mean
16
Standard deviations
Mean 3 children
Women with 0 or 10 children are far from the mean
17
Standard Deviations in the DHS

• Table 11.11 Nutritional status of children
• Percentage of children under five years
  classified as malnourished according to three
  anthropometric indices of nutritional status
  height-for-age, by residence, Ethiopia 2005

Therefore, 47 of Ethiopian children are 2SD or
more below the average height when compared to
children their age from all over the world. This
indicates that many Ethiopian children are not
growing at a healthy rate.
18
A New Neighbourhood
10,000
1,000
1,000
1,000
5,000
900
100
300
700
500