Deciles and Percentiles

1
Deciles and Percentiles

• Deciles If data is ordered and divided into 10
  parts, then cut points are called Deciles
• Percentiles If data is ordered and divided into
  100 parts, then cut points are called
  Percentiles. 25th percentile is the Q1, 50th
  percentile is the Median (Q2) and the 75th
  percentile of the data is Q3.
• Suppose PC ((n1)/100)p, where nnumber of
  observations and p is the desired percentile. If
  PC is an integer than pth percentile of a data
  set is the (PC)th observation of the ordered set
  of that data. Otherwise let PI be the integer
  part of PC and f be the fractional part of PC.
  Then pth percentile OI (OII -OI)xf where OI
  is the (PI)th observation of the ordered set of
  data and OII is the (PI 1)th observation of the
  ordered set of data.
• For example, Consider the following ordered set
  of data 3, 5, 7, 8, 9, 11, 13, 15.
• PC (9/100)p
• For 25 th percentile, PC2.25 (not an integer),
  then
• 25th percentile 5 (7-5)x.25 5.5

2
Coefficient of Variation

• Coefficient of Variation The standard deviation
  of data divided by its mean. It is usually
  expressed in percent.
• Coefficient of Variation

3
Five Number Summary

• Five Number Summary The five number summary of a
  distribution consists of the smallest (Minimum)
  observation, the first quartile (Q1), the
  median(Q2), the third quartile, and the largest
  (Maximum) observation written in order from
  smallest to largest.
• Box Plot A box plot is a graph of the five
  number summary. The central box spans the
  quartiles. A line within the box marks the
  median. Lines extending above and below the box
  mark the smallest and the largest observations
  (i.e., the range). Outlying samples may be
  additionally plotted outside the range.

4
Boxplot
Distribution of Age in Month
5
Side by Side Boxplot
Trt 3
Trt 2
Trt 1
6
Choosing a Summary

• The five number summary is usually better than
  the mean and standard deviation for describing a
  skewed distribution or a distribution with
  extreme outliers. The mean and standard deviation
  are reasonable for symmetric distributions that
  are free of outliers.
• In real life we cant always expect symmetry of
  the data. Its a common practice to include
  number of observations (n), mean, median,
  standard deviation, and range as common for data
  summarization purpose. We can include other
  summary statistics like Q1, Q3, Coefficient of
  variation if it is considered to be important for
  describing data.

7
Shape of Data

• Shape of data is measured by
• Skewness
• Kurtosis

8
Skewness

• Measures of asymmetry of data
• Positive or right skewed Longer right tail
• Negative or left skewed Longer left tail

9
Kurtosis Formula
10
Kurtosis
Kurtosis relates to the relative flatness or
peakedness of a distribution. A standard normal
distribution (blue line µ 0 ? 1) has
kurtosis 0. A distribution like that
illustrated with the red curve has kurtosis gt 0
with a lower peak relative to its tails.
11
Summary of the Variable Age in the given data
set
12
Summary of the Variable Age in the given data
set
13
Brief concept of Statistical Softwares

• There are many softwares to perform statistical
  analysis and visualization of data. Some of them
  are SAS (System for Statistical Analysis),
  S-plus, R, Matlab, Minitab, BMDP, Stata, SPSS,
  StatXact, Statistica, LISREL, JMP, GLIM, HIL, MS
  Excel etc. We will discuss MS Excel and SPSS in
  brief.
• Some useful websites for more information of
  statistical softwares-
• http//www.galaxy.gmu.edu/papers/astr1.html
• http//ourworld.compuserve.com/homepages/Rainer_Wu
  erlaender/statsoft.htmarchiv
• http//www.R-project.org

14
Microsoft Excel

• A Spreadsheet Application. It features
  calculation, graphing tools, pivot tables and a
  macro programming language called VBA (Visual
  Basic for Applications).
• There are many versions of MS-Excel. Excel XP,
  Excel 2003, Excel 2007 are capable of performing
  a number of statistical analyses.
• Starting MS Excel Double click on the Microsoft
  Excel icon on the desktop or Click on Start --gt
  Programs --gt Microsoft Excel.
• Worksheet Consists of a multiple grid of cells
  with numbered rows down the page and
  alphabetically-tilted columns across the page.
  Each cell is referenced by its coordinates. For
  example, A3 is used to refer to the cell in
  column A and row 3. B10B20 is used to refer to
  the range of cells in column B and rows 10
  through 20.

15
Microsoft Excel
Opening a document File ? Open (From a existing
workbook). Change the directory area or drive to
look for file in other locations.
Creating a new workbook File?New?Blank Document
Saving a File File?Save
Selecting more than one cell Click on a cell
e.g. A1), then hold the Shift key and click on
another (e.g. D4) to select cells between and A1
and D4 or Click on a cell and drag the mouse
across the desired range.

• Creating Formulas 1. Click the cell that you
  want to enter the formula, 2. Type (an equal
  sign), 3. Click the Function Button, 4.
  Select the formula you want and step through the
  on-screen instructions.

16
Microsoft Excel

• Entering Date and Time Dates are stored as
  MM/DD/YYYY. No need to enter in that format. For
  example, Excel will recognize jan 9 or jan-9 as
  1/9/2007 and jan 9, 1999 as 1/9/1999. To enter
  todays date, press Ctrl and together. Use a or
  p to indicate am or pm. For example, 830 p is
  interpreted as 830 pm. To enter current time,
  press Ctrl and together.
• Copy and Paste all cells in a Sheet CtrlA for
  selecting, Ctrl C for copying and CtrlV for
  Pasting.
• Sorting Data ? Sort? Sort By
• Descriptive Statistics and other Statistical
  methods Tools?Data Analysis? Statistical method.
  If Data Analysis is not available then click on
  Tools? Add-Ins and then select Analysis ToolPack
  and Analysis toolPack-Vba

17
Microsoft Excel
Statistical and Mathematical Function Start
with sign and then select function from
function wizard
Inserting a Chart Click on Chart Wizard (or
Insert?Chart), select chart, give, Input data
range, Update the Chart options, and Select
output range/ Worksheet.
Importing Data in Excel File ?open ?FileType
?Click on File? Choose Option ( Delimited/Fixed
Width) ?Choose Options (Tab/ Semicolon/ Comma/
Space/ Other) ? Finish.
Limitations Excel uses algorithms that are
vulnerable to rounding and truncation errors and
may produce inaccurate results in extreme cases.
18
Statistics Packagefor the Social Science (SPSS)
A general purpose statistical package SPSS is
widely used in the social sciences, particularly
in sociology and psychology.
SPSS can import data from almost any type of file
to generate tabulated reports, plots of
distributions and trends, descriptive statistics,
and complex statistical analyzes.
Starting SPSS Double Click on SPSS on desktop or
Program?SPSS.
Opening a SPSS file File?Open
MENUS AND TOOLBARS
Data Editor
Various pull-down menus appear at the top of the
Data Editor window. These pull-down menus are at
the heart of using SPSSWIN. The Data Editor menu
items (with some of the uses of the menu) are
19
Statistics Packagefor the Social Science (SPSS)
MENUS AND TOOLBARS
FILE used to open and save data files EDIT
used to copy and paste data values used to
find data in a file insert variables and
cases OPTIONS allows the user to set general
preferences as well as the setup for the
Navigator, Charts, etc. VIEW user can
change toolbars value labels can be seen in
cells instead of data values DATA select,
sort or weight cases merge files
TRANSFORM Compute new variables, recode
variables, etc.
20
Statistics Packagefor the Social Science (SPSS)

• MENUS AND TOOLBARS
• ANALYZE perform various statistical procedures
• GRAPHS create bar and pie charts, etc
• UTILITIES add comments to accompany data file
  (and other, advanced features)
• ADD-ons these are features not currently
  installed (advanced statistical procedures)
• WINDOW switch between data, syntax and
  navigator windows
• HELP to access SPSSWIN Help information

21
Statistics Packagefor the Social Science (SPSS)
MENUS AND TOOLBARS
Navigator (Output) Menus
When statistical procedures are run or charts are
created, the output will appear in the Navigator
window. The Navigator window contains many of the
pull-down menus found in the Data Editor window.
Some of the important menus in the Navigator
window include INSERT used to insert page
breaks, titles, charts, etc. FORMAT for
changing the alignment of a particular portion of
the output
22
Statistics Packagefor the Social Science (SPSS)
Formatting Toolbar
When a table has been created by a statistical
procedure, the user can edit the table to create
a desired look or add/delete information.
Beginning with version 14.0, the user has a
choice of editing the table in the Output or
opening it in a separate Pivot Table (DEFINE!)
window. Various pulldown menus are activated when
the user double clicks on the table. These
include EDIT undo and redo a pivot, select a
table or table body (e.g., to change the
font) INSERT used to insert titles, captions
and footnotes PIVOT used to perform a pivot of
the row and column variables FORMAT various
modifications can be made to tables and cells
23
Statistics Packagefor the Social Science (SPSS)
Additional menus
CHART EDITOR used to edit a graph SYNTAX
EDITOR used to edit the text in a syntax window
Show or hide a toolbar Click on VIEW ?
TOOLBARS ? ??to show it/ to hide it
Move a toolbar Click on the toolbar (but not
on one of the pushbuttons) and then drag the
toolbar to its new location Customize a
toolbar Click on VIEW ? TOOLBARS ?
CUSTOMIZE
24
Statistics Packagefor the Social Science (SPSS)
Importing data from an EXCEL spreadsheet Data
from an Excel spreadsheet can be imported into
SPSSWIN as follows 1. In SPSSWIN click on FILE ?
OPEN ? DATA. The OPEN DATA FILE Dialog Box will
appear. 2. Locate the file of interest Use the
"Look In" pull-down list to identify the folder
containing the Excel file of interest 3. From the
FILE TYPE pull down menu select EXCEL (.xls).
4. Click on the file name of interest and click
on OPEN or simply double-click on the file name.
5. Keep the box checked that reads "Read variable
names from the first row of data". This presumes
that the first row of the Excel data file
contains variable names in the first row. If the
data resided in a different worksheet in the
Excel file, this would need to be entered.
6. Click on OK. The Excel data file will now
appear in the SPSSWIN Data Editor.
25
Statistics Packagefor the Social Science (SPSS)
Importing data from an EXCEL spreadsheet
7. The former EXCEL spreadsheet can now be saved
as an SPSS file (FILE ? SAVE AS) and is ready to
be used in analyses. Typically, you would label
variable and values, and define missing values.
Importing an Access table SPSSWIN does not offer
a direct import for Access tables. Therefore, we
must follow these steps 1. Open the Access
file 2. Open the data table 3. Save the data as
an Excel file 4. Follow the steps outlined in the
data import from Excel Spreadsheet to SPSSWIN.
Importing Text Files into SPSSWIN
Text data points typically are separated (or
delimited) by tabs or commas. Sometimes they
can be of fixed format.
26
Statistics Packagefor the Social Science (SPSS)

• Importing tab-delimited data
• In SPSSWIN click on FILE ? OPEN ? DATA. Look in
  the appropriate location for the text file. Then
  select Text from Files of type Click on the
  file name and then click on Open. You will see
  the Text Import Wizard step 1 of 6 dialog box.
• You will now have an SPSS data file containing
  the former tab-delimited data. You simply need to
  add variable and value labels and define missing
  values.
• Exporting Data to Excel
• click on FILE ? SAVE AS. Click on the File Name
  for the file to be exported. For the Save as
  Type select from the pull-down menu Excel
  (.xls). You will notice the checkbox for write
  variable names to spreadsheet. Leave this
  checked as you will want the variable names to be
  in the first row of each column in the Excel
  spreadsheet. Finally, click on Save.

27
Statistics Packagefor the Social Science (SPSS)

• Running the FREQUENCIES procedure
• 1. Open the data file (from the menus, click on
  FILE ? OPEN ? DATA) of interest.
• 2. From the menus, click on ANALYZE ?
  DESCRIPTIVE STATISTICS ? FREQUENCIES
• 3. The FREQUENCIES Dialog Box will appear. In
  the left-hand box will be a listing ("source
  variable list") of all the variables that have
  been defined in the data file. The first step is
  identifying the variable(s) for which you want to
  run a frequency analysis. Click on a variable
  name(s). Then click the gt pushbutton. The
  variable name(s) will now appear in the
  VARIABLES box ("selected variable list").
  Repeat these steps for each variable of interest.
• 4. If all that is being requested is a
  frequency table showing count, percentages (raw,
  adjusted and cumulative), then click on OK.

28
Statistics Packagefor the Social Science (SPSS)

• Requesting STATISTICS
• Descriptive and summary STATISTICS can be
  requested for numeric variables. To request
  Statistics
• 1. From the FREQUENCIES Dialog Box, click on the
  STATISTICS... pushbutton.
• 2. This will bring up the FREQUENCIES
  STATISTICS Dialog Box.
• 3. The STATISTICS Dialog Box offers the user a
  variety of choices
• DESCRIPTIVES
• The DESCRIPTIVES procedure can be used to
  generate descriptive statistics (click on ANALYZE
  ? DESCRIPTIVE STATISTICS ? DESCRIPTIVES). The
  procedure offers many of the same statistics as
  the FREQUENCIES procedure, but without generating
  frequency analysis tables.

29
Statistics Packagefor the Social Science (SPSS)

• Requesting CHARTS
• One can request a chart (graph) to be created
  for a variable or variables included in a
  FREQUENCIES procedure.
• 1. In the FREQUENCIES Dialog box click on
  CHARTS.
• 2. The FREQUENCIES CHARTS Dialog box will
  appear. Choose the intended chart (e.g. Bar
  diagram, Pie chart, histogram.
• Pasting charts into Word
• 1. Click on the chart.
• 2. Click on the pulldown menu EDIT ? COPY
  OBJECTS
• 3. Go to the Word document in which the chart is
  to be embedded. Click on EDIT ? PASTE SPECIAL
• 4. Select Formatted Text (RTF) and then click on
  OK
• 5. Enlarge the graph to a desired size by
  dragging one or more of the black squares along
  the perimeter (if the black squares are not
  visible, click once on the graph).

30
Statistics Packagefor the Social Science (SPSS)

• BASIC STATISTICAL PROCEDURES CROSSTABS
• 1. From the ANALYZE pull-down menu, click on
  DESCRIPTIVE STATISTICS ? CROSSTABS.
• 2. The CROSSTABS Dialog Box will then open.
• 3. From the variable selection box on the left
  click on a variable you wish to designate as the
  Row variable. The values (codes) for the Row
  variable make up the rows of the crosstabs table.
  Click on the arrow (gt) button for Row(s). Next,
  click on a different variable you wish to
  designate as the Column variable. The values
  (codes) for the Column variable make up the
  columns of the crosstabstable. Click on the arrow
  (gt) button for Column(s).
• 4. You can specify more than one variable in the
  Row(s) and/or Column(s). A cross table will be
  generated for each combination of Row and Column
  variables

31
Statistics Packagefor the Social Science (SPSS)

• Limitations SPSS users have less control over
  data manipulation and statistical output than
  other statistical packages such as SAS, Stata
  etc.
• SPSS is a good first statistical package to
  perform quantitative research in social science
  because it is easy to use and because it can be a
  good starting point to learn more advanced
  statistical packages.

32
Normal Distribution
A density curve describes the overall pattern of
a distribution. The total area under the curve is
always 1.
A distribution is normal if its density curve is
symmetric, single-peaked and bell-shaped.
Mean, Median, and mode are same for a normal
distribution
A normal distribution can be described if we know
their mean and standard deviation. The
probability density function of a normal variable
with mean µ and standard deviation s can be
expressed as,
Normality and independence of the data are two
very important assumptions for most statistical
methods
33
Normal Distribution
If we know µ and s, we know every thing about the
normal distribution.
Total area under the curve is 1
s
2s
µ
34
Normal Distribution
The 68-95-99.7 Rule
In the normal distribution with mean µ and
standard deviation s
68 of the observations fall within s of the mean
µ.
95 of the observations fall within 2s of the
mean µ.
99.7 of the observations fall within 3s of the
mean µ.
s
s
3s
2s
2s
3s
35
Normal Density Plot
A sample of 100 observations from a normal
distribution with mean 0 and standard deviation 1.
68
95
36
Normal Distribution
Standardizing and z-Scores
If x is an observation from a distribution that
has mean µ and standard deviation s, the
standardized value of x is
A standardized value is often called a z-score.
If x is normal distribution with mean µ and
standard deviation s, then z is a standard normal
variable with mean 0 and standard deviation 1.
37
Normal Distribution
Let x1, x2, ., xn be n random variables each
with mean µ and standard deviation s, then sum of
all of them ?xi be also a normal with mean nµ and
standard deviation svn. The distribution of mean
is also a normal with mean µ and standard
deviation s/vn.
The standardized score of the mean is,
The mean of this standardized random variable is
0 and standard deviation is 1.
38
Assessing the normality of data

• Most statistical methods assume that data are
  from a normal population. So its important to
  test the normality of the data.
• Normal quantile plots
• If the points on a normal quantile plot lie
  close to diagonal line, the plot indicates that
  the data are normal. Otherwise, it indicates
  departure from normality. Points far away from
  the overall pattern indicates outliers. Minor
  wiggles can be overlooked. We will see normal
  quantile plots in next two slides.
• Shapiro-Wilk W statistics, Kolmogorov-Smirnov
  (K-S) tests etc are being used for testing
  normality of the data.
• To perform a K-S Test for Normality in SPSS,
  Analyzegt Nonparametric Tests gt 1 Sample K-S.
  Choose OK after selecting variable (s).
• To perform Shapiro-Wilk test of normality in SAS
  use procedure Univariate.

39
Normal quantile plot
q-q plot 100 sample observations from a normal
distribution with mean 0 and standard deviation 1
40
Normal quantile plot
41
Population and Sample

• Population The entire collection of individuals
  or measurements about which information is
  desired e.g. Average height of 5-year old
  children in USA.
• Sample A subset of the population selected for
  study. Primary objective is to create a subset of
  population whose center, spread and shape are as
  close as that of population. There are many
  methods of sampling. Random sampling, stratified
  sampling, systematic sampling, cluster sampling,
  multistage sampling, area sampling, qoata
  sampling etc.
• Random Sample A simple random sample of size n
  from a population is a subset of n elements from
  that population where the subset is chosen in
  such a way that every possible unit of population
  has the same chance of being selected.
• Example Consider a population of 5 numbers (1,
  2, 3, 4, 5). How many random sample (without
  replacement) of size 2 can we draw from this
  population ?
• (1,2), (1,3), (1, 4), (1, 5), (2, 3), (2, 4),
  (2, 5), (3,4), (3,5), (4,5)

42
Population and Sample

• Why do we need randomness in sampling?
• It reduces the possibility of subjective and
  other biases.
• Mean and variance of a random sample is an
  unbiased estimate of the population mean and
  variance respectively.
• Population mean of the five numbers in previous
  slide is 3. Averages of 10 samples of sizes 2 are
  1.5, 2, 2.5, 3, 2.5, 3, 3.5, 3.5, 4, 4.5. Mean of
  this 10 averages (1.5 2 2.5 3 2.5 3
  3.5 3.5 4 4.5)/10 3 which is the same as the
  population mean.

43
Parameter and Statistic

• Parameter Any statistical characteristic of a
  population. Population mean, population median,
  population standard deviation are examples of
  parameters.
• Statistic Any statistical characteristic of a
  sample. Sample mean, sample median, sample
  standard deviation are some examples of
  statistics.
• Statistical Issue Describing population through
  census or making inference from sample by
  estimating the value of the parameter using
  statistic.

44
Census and Inference

• Census Complete enumeration of population units.
• Statistical Inference We sample the population
  (in a manner to ensure that the sample correctly
  represents the population) and then take
  measurements on our sample and infer (or
  generalize) back to the population.
• Example We may want to know the average height
  of all adults (over 18 years old) in the U.S. Our
  population is then all adults over 18 years of
  age. If we were to census, we would measure every
  adult and then compute the average. By using
  statistics, we can take a random sample of adults
  over 18 years of age, measure their average
  height, and then infer that the average height of
  the total population is close to'' the average
  height of our sample.

45
Univariate, Bivariate, and Multivariate Data

• Depending on how many variables we are measuring
  on the individuals or objects in our sample, we
  will have one of the three following types of
  data sets
• Univariate Measurements made on only one
  variable per observation.
• Bivariate Measurements made on two variables per
  observation.
• Multivariate Measurements made on more than two
  variables per observation.

46
Examining Relationship

• Response Variable Measures the outcome of the
  study, treatment, or experimental manipulation.
• Explanatory Variable Explains or influences
  changes in a response variable. This is also
  known as an independent variable or prediction
  variable.
• Scatter plot Shows the relationship between two
  quantitative variables measured on the same
  individuals. We look for the overall pattern and
  striking deviations from that pattern. Overall
  pattern of a scatter plot by the form, direction,
  and strength of the relationship.
• Positive relation Association in the same
  direction
• Negative relation Association in the opposite
  direction

47
Examining Relationship

• Form Linear relationship, Curve linear
  relationship, Cluster etc.
• Linear Relationship Points of the scatter plot
  show a straight-line pattern.
• Strength of the Relationship is determined by
  how close the points in the scatter plot lie to a
  simple form such as line.
• Correlation measures the strength between two
  variables.
• We will learn more about the relationship of
  variables later.

48
Proportion

• Proportion In many cases, it is appropriate to
  summarize a group of independent observations by
  the number of observations in the group that
  represent one of two outcomes.
• Consider a variable X with two outcomes 1 and 0
  for happening and not happening of some events
  correspondingly. Let p be the probability that
  the event happens then pProb(X1).
• Suppose, we want to estimate of the proportion of
  the Patients coming to duPont having some
  particular disease. To estimate this proportion
  (population), we need to take a sample of size n
  and examine if the patient is bearing that
  particular disease. Then the estimated proportion
  is,

49
Proportion

• For large n, the sampling distribution of is
  approximately normal with mean P (Population
  Proportion) and the standard deviation
• If probability of happening one event is p, then
  probability of not happening of the same event is
  1-p and total probability is 1.
• What is the difference between proportion and a
  sample mean?
• If X takes two values 0 or 1 and p is the
  proportion of happening an event i. e.
  pprob(x1), then proportion is the same as
  sample mean.

50
Binomial Distribution

• Let us consider an experiment with two outcomes
  success (s) and failure (F) for each subject and
  the experiment was done for n subjects. The
  sequence of S and F can be arranged as follows-
• SSFSFFFSSFSF
• where there are x success out of n trial. Then
  the probability distribution of x can written as

The mean and variance of x are np and np(1-p).
51
Binomial Distribution

• If p1/2, then Binomial distribution is
  symmetric.

52
Useful Website(s)

• http//www.cas.lancs.ac.uk/glossary_v1.1/main.html