Introduction to Psychological Research Methods
A community living close to an industrial area has raised concerns about contaminants from industry causing health problems in local families. Based on current research, the public health guidelines state that children should not have blood lead levels above 10 micrograms per decilitre, because elevated lead levels may cause long-term cognitive impairment. As part of the investigation, parents of all children in the 2- to 6-year-old age range who live within a 10km radius of the industrial area are contacted and strongly encouraged to have their children assessed. Lead levels in the children’s blood of are measured. Blood samples are taken by a qualified phlebotomist at children’s homes. A psychologist is employed to assess the children’s level of cognitive development. Each child is assessed individually at the psychologist’s office, using a standardised developmental scale. For each child, their attained score is compared to the score expected for children their age (in years and months), based on norms for the test gathered on a nationally representative sample of 200 children per age group in 2009. A discrepancy score is calculated, which indicates how many months ahead of, or behind, the developmental norm for their age each child is. Positive scores indicate being ahead of same-age peers. Negative scores indicate developmental delay. The table below shows the lead levels (in g/dL) of the 20 children tested and their developmental discrepancy scores (in months).For statistical calculations, please round your final answers to two (2) decimal places.
- What is the independent variable in this study? What is the dependent variable in this study? Explain your answer.
A lead level is the independent variable while the developmental score is the dependable variable. This is because the cognitive developmental score seems to increase or decrease depending on the level of lead present.Table 1. Complete the following table:
- Calculate the Mean, Standard deviation and Standard error of the mean for lead levels and development scores.
- Calculate single sample t to compare the children’s lead levels to the recommended maximum of 10 micrograms per deciliter. Calculate single sample t to compare the children’s developmental discrepancy scores to a mean of 0 (indicating normal rate of cognitive development). Complete the following table. Please show your working on a separate sheet.
- Write the results of the t-tests in APA format.The mean lead level was 10.65, with a standard deviation of 2.3. This was statistically not significantly higher than the maximum recommended level of 10, t(19) = 1.26, p < .05.
The mean developmental score was -7.95, with a standard deviation of 8.15. This was statistically not significantly higher than the maximum recommended level of 0, t(19) = -4.36, p < .05.
- What conclusions would you feed back to the investigation about the lead levels of these children compared to the recommended maximum, and their level of cognitive development compared to the norm?
The lead levels in the children are not very different from the recommend maximum and the developmental score are not very different from the norm Thus, these results are effective in trying to show the relationship between the lead level and cognitive development.
- Draw a scatterplot showing the relationship between blood lead levels and developmental discrepancy scores. You may do this by hand on graph paper or using Excel if you prefer.
- Calculate Pearson’s r for the association between blood lead levels and developmental discrepancy scores. Complete the following table. Please show your working on a separate sheet.
- Write the result of this Pearson’s r in APA format.The correlation between lead levels and developmental scores is statistically significant, r(18) = 0.46, p > .05.
Overall, there was a negative correlation between lead and cognitive development. An increase in lead was correlated with a decrease in developmental score.
- What are the main methodological STRENGTHS of this study? Explain briefly why they are strengths.
- Sample collection was done by a qualified phlebotomist and cognitive development measured by a psychologist.
- As the data is collected in the children’s natural environment, it provides validity for the study.
- The developmental scores are compared to norms for the test gathered on a nationally representative sample of 200 children per age group in 2009
- What are the main methodological WEAKNESSES of this study? Explain briefly why they are weaknesses and how you could improve them.
- Sample size is relatively small hence generalizing it to a larger population would introduce errors. This can be improved by increasing the number of children sampled within a 10 km radius.
- The sampling approach may be biased. This is because not all parents contacted will show up.
- The study only establishes a relationship and not a cause for the cognitive impairment.
PSY173Introduction to Psychological Research Methods
Statistical Analysis Exercise3
Statistical Formulae
Central Tendency
Mean:
Variability:
Variance:
Standard deviation:
Standard error of the mean:
Single-sample t-test
Step 1: Calculate tobtained
Where comparison value or mean of comparison population
Step 2: Use table to find tcritical
df = n –1
Step 3: Decide whether or not t is significant
If tobtained> tcritical reject null hypothesis (H0), difference is significant
If tobtained</= tcritical retain null hypothesis (H0), difference is not significant
Pearson’s r (Correlation coefficient)
Step 1: Calculate r
Raw score formula:
Step 2: Use table to find rcritical
df = n-2
Step 3: Decide whether or not robtained is significant.
If robtained> rcritical reject null hypothesis (H0), correlation is significant
If robtained</= rcritical retain null hypothesis (H0), correlation is not significant
