# ISA 225

## Notes for Principles of Business Analytics - ISA 225

Prepping resources:

## Review of Statistical Inference

• Population:
• N Size
• μ Mean
• 𝜎 Standard Deviation
• P Ratio (proportion)
• Sample:
• n Size
• Ratio (proportion) (sample portion size / sample size)
• Mean
• Portion Size
$Z-score = {sampleRatio - populationRatio \over \sqrt{populationRatio(1-populationRatio) \over sampleSize} }$ $Z-score = {sampleMean - populationMean \over{populationStandardDeviation \over \sqrt{sampleSize}} }$ $Z-score = {p̂ - P \over \sqrt{p̂(1-p̂) \over n} }$ $Z-score = {x̄ - μ \over{𝜎 \over \sqrt{n}} }$

### Checks

• Checks:
• Independent:
• ✅ Randomly selected
• ✅ 10% condition: n < 10% of P
• Normality:
• ✅ Success/Failure:
• ✅ np >= 10
• ✅ n(1-p) >= 10
• ✅ Expect at least 10 success and 10 failures

### Common Z-Values

α Confidence Level Z*
0.1 90% 1.645
0.05 95% 1.960 ≈ 2
0.01 99% 2.576

### Determining Sample Size

$sampleSize = {sampleRatio(1-sampleRatio) \times\left({zIndex \over{marginOfError}}\right)^2 }$ $n = {p̂(1-p̂) \times\left({Z^* \over{ME}}\right)^2 }$ $sampleSize = {\left({zIndex \times{ populationStandardDeviation} \over{marginOfError}}\right)^2}$ $n = {\left({Z^* \times{𝜎} \over{ME}}\right)^2}$ $sampleSize = {\left({zIndex \times{ sampleStandardDeviation} \over{marginOfError}}\right)^2}$ $n = {\left({Z^* \times{s} \over{ME}}\right)^2}$

The t-distribution describes the statistical properties of sample means that are estimated from small samples; the standard normal distribution is used for large samples.

## Hypothesis test for population proportion and mean

A hypothesis is a claim about a population parameter (proportion, mean)

Steps to compute a hypothesis test:

1. State hypothesis
2. Calculate test statistics
3. Find p-value
4. Make conclusions based on p-value

The null hypothesis Ho, is the starting assumption (nothing has changed).

$H_o: populationParameter = claimedValue$

The alternative hypothesis, or Ha is a claim the population parameter value differs from the null hypothesis. It can take these different forms depending on what you want to test (H_a):

Left-tailed hypothesis test:
$$H_a: populationParameter \lt claimedValue$$

Right-tailed hypothesis test:
$$H_a: populationParameter \gt claimedValue$$

Two-tailed hypothesis test:
$$H_a: populationParameter \neq claimedValue$$

### Step 2: Calculate the Test Statistics

• Test statistics about population proportion P (One-prop Z-test)
• $Z-score = {p̂ - P \over \sqrt{p(1-p) \over n} }$
• Test statistics about population mean μ (One-sample test)
1. When the population 𝜎 Standard Deviation is known: (One-sample Z-test)
• $Z-score = {sampleMean - claimedValue \over{populationMean \over \sqrt{sampleSize}} }$
• $Z-score = {x̄ - μ \over{𝜎 \over \sqrt{n}} }$
2. When the population 𝜎 Standard Deviation is unknown (One-same t test / student t-test)
• $t = {sampleMean - claimedValue \over{sampleStdDev \over \sqrt{sampleSize}} }$
• $t = {x̄ - μ_0 \over{s \over \sqrt{n}} }$

### Step 4: Make conclusion based on p-value

Compare p-value with significance level α (always given before test). The smaller α, the more accurate the test is.

1. Type I errors, the null hypothesis is true, but we reject it (false negative)
2. Type II errors, the null hypothesis is false, but we fail to reject it (false positive)

If p-value < α, then reject null hypothesis, we have enough evidence to support Ha.

If p-value > α, then do not reject null hypothesis, we do not have enough evidence to support Ha.

## Comparing Two Population Parameters

### Two Sample t-test (comparing two population means)

1. State hypothesis
2. Check assumptions and calculate test statistics
3. Find p-value based on test statistics
4. Make conclusion based on p-value
$Z = {(ȳ_1 - ȳ_2) - (μ_1 - μ_2) \over\sqrt{ {𝜎_1^2 \over{ n_1 }} + {𝜎_2^2 \over{n_2}} } }$

Since population standard deviations are unknown, we use the standard errors instead:

$t = {(ȳ_1 - ȳ_2) - (μ_1 - μ_2) \over\sqrt{ {s_1^2 \over{ n_1 }} + {s_2^2 \over{n_2}} } }$

### Confidence Interval for Difference between Two Population Means

Two sample Z-interval (when $$𝜎_1$$ and $$𝜎_2$$ are known)

${(ȳ_1 - ȳ_2) \pm Z^* * \sqrt{ {𝜎_1^2 \over{ n_1 }} + {𝜎_2^2 \over{n_2}} } }$

Two sample Z-interval (when $$𝜎_1$$ and $$𝜎_2$$ are unknown)

${(ȳ_1 - ȳ_2) \pm t^* * \sqrt{ {s_1^2 \over{ n_1 }} + {s_2^2 \over{n_2}} } }$

The $$t^*_{df, a/2}$$ here depends on the confidence level 100(1-α)% and the calculated df.

## Chi-Square Tests

• One variable?
• Goodness of Fit Test
• H0: model fits data
• Ha: model does not fit data
• Two variables?
• Test for independence
• H0: variables are independent
• Ha: variables are not independent

### Goodness-of-Fit Tests (one variable)

A χ2 goodness of fit test is applied when you have one categorical variable from a single population.

1. State the hypothesis:
• H0: model fits. (hypothesized model fits the sample we collected)
• Ha: model doesn’t fit. (hypothesized model doesn’t fit the sample we collected)
2. Assumptions and Test Statistics:
• Assumptions:
• Counted Data Condition – The data must be counts for the categories of a single categorical variable.
• Independence Assumption – The counts should be independent of each other.
• Randomization Condition – The counted individuals should be a random sample of the population.
• Sample Size Assumption – We expect at least 5 individuals per cell. - Test statistics:
• ${\chi^2 = \sum_{allCells} {(Obs - Exp)^2\over{Exp} } }$
3. Find p-value based on the test statistics
• Df= (#cells -1), use the χ2 table, fix the line of df, then with the test statistics to find the corresponding p-value, which is the right-tail probability of the test statistics.
• (or by technology) p-value= P(χ2 > test statistics)
4. Make Conclusions based the p-value
• If p-value < α, reject the H0, which means the hypothesized model doesn’t fit the sampled data.
• If p-value > α, fail to reject H0, we do not have significant evidence to say the model doesn’t fit the sampled data.

### Chi-Square test for Independence (two variables)

1. State the hypothesis:
• H0: variables are independent.
• Ha: variables are not independent.
2. Assumptions and Test Statistics:
• Assumptions:
• Counted Data Condition – The data must be counts for the categories of a single categorical variable.
• Randomization Condition – The counted individuals should be a random sample of the population.
• Sample Size Assumption – We expect at least 5 individuals per cell. - Test statistics:
• ${\chi^2 = \sum_{allCells} {(Obs - Exp)^2\over{Exp} } }$
• Assuming H0 is true, which means that the variables are independent.
• ${Exp_{ij} = {totalRow_i \times totalCol_i}\over{tableTotal} }$
3. Find p-value based on the test statistics
• Df= (# of rows -1)×(# of cols-1), use the χ2 table, fix the line of df, then with the test statistics to find the corresponding p-value, which is the right-tail probability of the test statistics.
• p-value= P(χ2 > test statistics)
4. Make Conclusions based the p-value
• If p-value < α, reject the H0, which means the two variables are not independent.
• If p-value > α, fail to reject H0, we do not have significant evidence to say the two variables are not independent.

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