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Unpublished: BBS, WHO, IMF, FAO, ICDDR,B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

Three things required

Range
No. of classes (k) &
Class Interval (CI)

Let k or CI & find the other

$CI = \frac{Range}{\text{Number of classes (k)}}$
$\text{Number of classes, k}= \frac{Range}{\text{CI}}$
Sturges Method: $k = 1 + 3.322 \space logN$; where N = no. of observations

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?

Draw Suitable Chart

Favorite colors of 30 individuals are noted down.

  Brown Red   Pink  Green Green Green Brown Pink  Brown Red   
  
Brown Red   Green Pink  White Red   Brown Green White Brown 

White Brown Pink  Red   White Brown Green Red   Pink  Red

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