Statistics I: Collection, Organization & Presentation of Data

Abdullah Al Mahmud

Data Collection

Types of Data

  • Qualitative
  • Quantitative

Sources of Data

Primary: Obtained directly (not collected from someone else)

  • Secondary: Using pre-collected data from someone else/some organization
  • Example
  • A researcher buys data from BMD to build a model of rainfall behavior
  • A researcher runs an experiment to measure speed of light using a novel technique.
  • A researcher makes use of the data generated by the one in example 2

Method of Data Collection

  • Direct personal Inquiry
  • Indirect oral inquiry
  • Mail
  • Telephone etc.
  • Each method has its own advantages and disadvantages;

Sources of Secondary Data

  • Published: Journal, Newspaper etc.
  • Unpublished: BBS, WHO, IMF, FAO, ICDDR,B

DIsadvantages of Secondary Data

  • Purpose might be different
  • Suitability
  • Reliability
  • Unit

Organizing Data

Tabluation

table

Data Classification

  • Geographical
  • Chronological
  • Quantitative
  • Qualitative

Example

Geographical

Country Bangladesh USA
GDP(m) 120 500

Chronological (Time series data)

Year 2015 2016
GDP(m) 120 500

Quantitative Classification

Income level 40,000-50,000 50,000-1,00,000
Frequency 120 34

Frequency Distribution

No. of classes

  • Sturges Method: \(k = 1 + 3.322 \space logN\); where N = no. of observations
  • Class Interval width = \({Range} \over {\text{Numbe rof classes}}\)

Graphs

Histogram

  • Inclusive vs exclusive
What does it tell us

Histogram (contd.)

Can these intervals be readily used?

(5-10); (10-15); (15-20)

(5-9); (10-14); (15-20)

If not, what should we do?

Stem and Leaf

  • key in stem and leaf plot
  • How to interpret stem and leaf plot
data <- c(16, 26, 12, 10, 27, 30, 14,  1, 25, 20)
stem(data)
## 
##   The decimal point is 1 digit(s) to the right of the |
## 
##   0 | 1
##   1 | 0246
##   2 | 0567
##   3 | 0

How to interpret cf and rf

Class Frequency Cumulative

Frequency (cf)		 Relative

Frequency (rf)		 Cumulative

Relative

Frequency (crf)
30-35 4 4 0.09 0.09
35-40 10 14 0.23 0.32
40-45 20 34 0.45 0.77
45-50 8 42 0.18 0.95
50-55 2 44 0.04 1
n=44 n=44

What Ogives tell us

Bar vs Pie

  • When to use which?
  • How to calculate angles?
  • Can we draw on 180 degrees?

Choose Diagram

year Sales ($)
1996 76
1997 58
1998 95
1999 85

Category Cost(Tk.)
House rent 10,000
Utility Bill 3,000
Telecom 2000

Frequency Polygon vs Frequency Curve

  • Curve: Smoothed corners

Bar Diagram vs Histogram