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MEASURING RELATIONSHIPS
Pearson r Product Moment Coefficient of Correlation
Mike Arieh P. Medina, PhD
Outline
• What is coefficient of correlation
• How to calculate a correlation
• Assumptions about correlation calculations
• Interpretation of the correlation
• Evaluating the significance of the relationship
The correlation coefficient
• Indicated by r
• Ranges from -1.00 to +1.00
– The value indicates the strength of the
relationship
– The closer to 0, the weaker the relationship
– The closer to +/- 1.00 the stronger the relationship
• The sign indicates whether the relationship is
positive or negative
Question
• Which of the following coefficient of
correlation indicates the strongest
relationship?
• A. + 0.45
• B. – 0.33
• C. + 0.58
• D. – 0.67
The Equation
Problem: Is no. of hospitals related to MMR?
City No. of Hospitals Maternal Mortality Rate
A 5 5
B 10 2
C 4 8
D 8 3
E 2 8
F 7 5
G 9 5
H 6 7
I 1 10
J 12 3
Hypothesis
• H0: There is no significant relationship
between no. of hospitals and MMR.
• H1: There is a significant relationship between
no. of hospitals and MMR.
0
2
4
6
8
10
12
0 2 4 6 8 10 12 14
MMR
Hospitals
Scatterplot of MMR vs. Hospitals
Types of Correlation
Positive Correlation Negative Correlation
No Correlation
Perfect Correlation
Types of Correlation
Strong Correlation Weak Correlation
Sample Problem
x y xy x2 y2
5 5 25 25 25
10 2 20 100 4
4 8 32 16 64
8 3 24 64 9
2 8 16 4 64
7 5 35 49 25
9 5 45 81 25
6 7 42 36 49
1 10 10 1 100
12 3 36 144 9
Σx = 64 Σy = 56 Σxy = 285 Σx2 = 520 Σy2 = 374
Computation
y
Computation
Computation
Correlation Assumptions
• Before you can compute for Pearson r make
sure that the following assumptions are
satisfied:
– The relationship is linear
– The variables are measured on an interval or ratio
scale
– The distribution of the variable is normal
– The distribution is continuous
Interpretation
Magnitude of r
Range of r Descriptive label for r
+/- 0.80 – 0.90 Very Strong
+/- 0.60 – 0.79 Strong
+/- 0.40 – 0.59 Moderate
+/- 0.20 – 0.39 Weak
+/- 0.00 – 0.19 Very Weak
Interpretation
• r2 = Coefficient of Determination
• Percent variance that is shared between variable x and
variable y
• 1- r2 = Coefficient of Alienation
• Percent variance that is not shared between variable x
and variable y
• Now, compute for the r2 of the previous example and
interpret.
Interpretation
Coefficient Value Interpretation
r 0.899
Very Strong negative relationship
between no. of hospitals and
MMR (the more hospitals in a city
the lesser its MMR)
r2 0.808
80.8% of the variation in MMR is
influenced by the variation in the
no. of hospitals
1-r2 0.192
19.2% of the variation in MMR is
influenced by factors other than
no. of hospitals
How do we know if it’s a significant
relationship? (Ex. at 0.05)
df = N-2
DOWNLOAD table of critical values of the Pearson Product-Moment correlation
Exercise
• Now, compute for the coefficient of
correlation using the population and weight of
waste hypothetical data.
• Population (thousands): 8, 6, 7, 4.5, 5
• Solid Waste (thousand tons): 2, 1.8, 1.5, 1.1, 1.25
• Create a scatterplot
• Interpret the results
• ANS: r = 0.858

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Measuring relationships

  • 1. MEASURING RELATIONSHIPS Pearson r Product Moment Coefficient of Correlation Mike Arieh P. Medina, PhD
  • 2. Outline • What is coefficient of correlation • How to calculate a correlation • Assumptions about correlation calculations • Interpretation of the correlation • Evaluating the significance of the relationship
  • 3. The correlation coefficient • Indicated by r • Ranges from -1.00 to +1.00 – The value indicates the strength of the relationship – The closer to 0, the weaker the relationship – The closer to +/- 1.00 the stronger the relationship • The sign indicates whether the relationship is positive or negative
  • 4. Question • Which of the following coefficient of correlation indicates the strongest relationship? • A. + 0.45 • B. – 0.33 • C. + 0.58 • D. – 0.67
  • 6. Problem: Is no. of hospitals related to MMR? City No. of Hospitals Maternal Mortality Rate A 5 5 B 10 2 C 4 8 D 8 3 E 2 8 F 7 5 G 9 5 H 6 7 I 1 10 J 12 3
  • 7. Hypothesis • H0: There is no significant relationship between no. of hospitals and MMR. • H1: There is a significant relationship between no. of hospitals and MMR.
  • 8. 0 2 4 6 8 10 12 0 2 4 6 8 10 12 14 MMR Hospitals Scatterplot of MMR vs. Hospitals
  • 9. Types of Correlation Positive Correlation Negative Correlation No Correlation Perfect Correlation
  • 10. Types of Correlation Strong Correlation Weak Correlation
  • 11. Sample Problem x y xy x2 y2 5 5 25 25 25 10 2 20 100 4 4 8 32 16 64 8 3 24 64 9 2 8 16 4 64 7 5 35 49 25 9 5 45 81 25 6 7 42 36 49 1 10 10 1 100 12 3 36 144 9 Σx = 64 Σy = 56 Σxy = 285 Σx2 = 520 Σy2 = 374
  • 15. Correlation Assumptions • Before you can compute for Pearson r make sure that the following assumptions are satisfied: – The relationship is linear – The variables are measured on an interval or ratio scale – The distribution of the variable is normal – The distribution is continuous
  • 16. Interpretation Magnitude of r Range of r Descriptive label for r +/- 0.80 – 0.90 Very Strong +/- 0.60 – 0.79 Strong +/- 0.40 – 0.59 Moderate +/- 0.20 – 0.39 Weak +/- 0.00 – 0.19 Very Weak
  • 17. Interpretation • r2 = Coefficient of Determination • Percent variance that is shared between variable x and variable y • 1- r2 = Coefficient of Alienation • Percent variance that is not shared between variable x and variable y • Now, compute for the r2 of the previous example and interpret.
  • 18. Interpretation Coefficient Value Interpretation r 0.899 Very Strong negative relationship between no. of hospitals and MMR (the more hospitals in a city the lesser its MMR) r2 0.808 80.8% of the variation in MMR is influenced by the variation in the no. of hospitals 1-r2 0.192 19.2% of the variation in MMR is influenced by factors other than no. of hospitals
  • 19. How do we know if it’s a significant relationship? (Ex. at 0.05) df = N-2 DOWNLOAD table of critical values of the Pearson Product-Moment correlation
  • 20. Exercise • Now, compute for the coefficient of correlation using the population and weight of waste hypothetical data. • Population (thousands): 8, 6, 7, 4.5, 5 • Solid Waste (thousand tons): 2, 1.8, 1.5, 1.1, 1.25 • Create a scatterplot • Interpret the results • ANS: r = 0.858