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#1
08-14-2012
 USMLE Forums Scout Steps History: Not yet Posts: 72 Threads: 8 Thanked 11 Times in 11 Posts Reputation: 21
Nominal vs Ordinal vs Interval vs Ratio

I am not able to recognize whether the data is Nominal, Ordinal, interval or ratio. I have read the definitions of each in HY, BRS, Kaplan and the Internet but I cannot identify the data type and miss the question on choose the statistical test.

Any help will be highly appreciated.

TIA

#2
08-15-2012
 USMLE Forums Newbie Steps History: Not yet Posts: 1 Threads: 0 Thanked 2 Times in 1 Post Reputation: 12

nominal-> no order, category labels (male or female, religious status, place of birth)
ordinal-> logical or ordered relationship across the value, some degree of difference between categories (no stress, mild stress, moderate stress, severe stress, extreme stress)
scale-> interval scale(unit) (10'F, 20'F) , no true zero point (eg., 0'F does not indicate that there is no temperature)
-> ratio scale , true zero point (eg., 0 year old at birth), length, weight and income
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#3
08-15-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 790 Threads: 76 Thanked 672 Times in 316 Posts Reputation: 690

i found this in another forums.....hope this shall clear the concept

1-"Interval variable" (values from where we can drive their "Mean")

e.g. measurements of B.P, Weight, Cholesterol levels etc

2-"Nominal variable" ( different categories or "GROUPS")

e.g.

*Gender ---is a Nominal that has Two Groups (Male and Female)
*AGE-------is a Nominal that we can have either TWO groups (Elderly and Middle age) or
THREE Groups(Elderly, Middle-age and Children) etc

*Race------a Nominal that can have many groups
Two groups (African Americans and Latino)
Three groups(African Americans, Latino and Asian) and so forth

e.g.
*"GENDER"(Men and Women)............................................ ........(Single Nominal)
*"GENDER"(Men and Women) n "RACE"(Caucasian and African)....(Two Nominals )

e.g we compare Blood Pressure measurements

*INTERVAL variable ("MEANS" of B.P measurements)
*NOMINAL variable ("GROUPS" )

so to compare MEANS of B.P. b/w two are more groups we do these tests

t-Test:

1 Interval (Blood pressure Means)
1 Nominal with "ONLY TWO GROUPS"

e.g. comparing the BP means b/w (Latinos) and (African Americans)

*for a t-Test it will be a wrong statement to say
e.g comparing B.P means b/w (Latinos), (African Americans) and (Asians)....*got 3 groups

Now what's the difference B/W t-Test and ANOVA

t-Test------>ONLY TWO Groups comparison
ANOVA------------>TWO Groups or more than Two Groups comparison

NOTE:these two will give u the Identical results if comparing "TWO Groups"

t-Test................... 1 Interval and 1 Nominal with TWO Groups
One-way ANOVA.....1 Interval and 1 Nominal with TWO Groups

and for the above reason they rather use One-way ANOVA when more than two groups are there to be compared e.g as above
comparing B.P means b/w (Latinos), (African Americans) and (Asians).......3 groups

Two-way ANOVA:
1 Interval
2 Nominal e.g. Gender (Men and Women) *AND Race (Caucasians and African Americans)

---->we do these tests to see If there is a Difference b/w groups ?
t-statistics (t-Test) and f-statistics(ANOVA) will be compared to get a p-value
If p-value = 0.05 or less -----it is said to be "Statistically Significant' and we say that there is a Difference in these Groups.

Chi-square:

we compare b/w "TWO NOMINAL" (with ANY # of GROUPS)

(Note; we are not using any Interval variable here , just the NOMINAL data)

best example...."Testing Drug Efficacy"
First Nominal........(New drug group) and (Placebo group)
Second Nominal ...(Recovered group) and(Not recovered group)

***I think this is all u have to do is recognize what kind of data is presented , how many Nominals, How many groups and If Interval variable is used or not?

1 Interval and 1 Nominal (Two Groups).......................t-Test or One-way ANOVA
1 Interval and 1 Nominal (More than Two groups)........One-way ANOVA
1 Interval and *2 Nominal (many groups)....................Two-way ANOVA
.....................*2 Nominal (any# of Groups).................Chi-square

Types of scales and Stats tests?
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Matched!!!
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#4
08-15-2012
 USMLE Forums Scout Steps History: Not yet Posts: 72 Threads: 8 Thanked 11 Times in 11 Posts Reputation: 21

Thanks anomali... this is what I was looking for... spelled out clearly....

To add to this few more Q....
• what is the difference between paired and unpaired t test?

How to recognize whether parametric or non parametric test is to be used?

I know what case-control and cohort studies are but I always mess up the question. I will be glad if someone could share the approach to this question.

TIA

 Tags Biostatistics-Epidemiology

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