College- en werkgroepaantekeningen bij Experimenteel en Correlationeel Onderzoek - UL
Collegeaantekeningen Experimenteel en Correlationeel Onderzoek, Psychologie Leiden
In de bijlagen vind je collegeaantekeningen van hoorcolleges en werkgroepen 1 t/m 7 van Experimenteel en Correlationeel Onderzoek, onderdeel van de bachelor Psychologie aan de Universiteit Leiden:
Lecture notes with Experimental and Correlational Research at the Leiden University - 2018/2019
Lecture 1
6/2/2019
-Correlation is about two variables being associated, but there is no evidence of causality.
-Causality however requires multiple factors: covariance (variables have an association), directionality(cause precedes effect (in time)) and internal validity(eliminate alternative explanations).
- Correlations can be displayed in scatterplots that show:
- Direction: positive or negative
- Strength: density of the points
- Shape: linear/nonlinear and homogeneous (one cluster) / heterogeneous (multiple clusters).
- Outliers
-Covariance (sxy): to measure the degree to which two variables vary together.
Formula: sxy = Σ(xi-x)(yi-y) / N-1
It provides us with information on the strength and direction of the association. The disadvantage is that the covariance is dependent on the unit of measurement of the variables.
-Pearson r is a standardized measure that describes the linear relationship between two quantitative variables, and always lies between -1 and +1.
Formulas: r = sxy / sxsy Alternative: r = Σzxzy / N-1
- Remember that a z-score is a standardized score that displays how many standard deviations a certain score is away from the mean.
Alternative correlational techniques:
- The Pearson r is the correlation coefficient that is most commonly used. There are alternatives:
quantitative + quantitative --> Pearson r
ordinal + ordinal --> Spearman’s rho (rs)
dichotomous(only two possible values)+ quantitative --> point-biserial correlation (rpb)
dichotomous + dichotomous --> phi coefficient (ϕ)
-Spearman’s rho (rs) describes relationship between two ordinal variables/ranked scores.
Formulas: xrank = N + 1 / 2 srank = √ (N(N+1) / 2)
rs = r on ranked data
Spearman’s rho is also an alternative to Pearson r in case of outliers and/or weak non-linearity.
-Point-biserial correlation describes relationship between quantitative and dichotomous variables. We use the Pearson correlation formula to calculate rpb: rpb = r
The sign of the correlation (+/-) depends on the way 0 and 1 are assigned to groups.
Relationship rpb and tindependent: rpb = Square root of t2 / t2 + df
-Phi-coefficient (ϕ) describes relationship between two dichotomous variables: ϕ
.....read more Workgroup notes with Experimental & Correlational Statistics at the Leiden University - 2018/2019
Week 1
Prep exercises
- Which combination of measurement levels is required for the use of the Pearson, Spearman, and point-biserial correlation respectively?
- Which formula is suitable for calculating the Pearson, Spearman, and point-biserial correlations?
- Which formula describes the relationship between rpb and tindep?
- Which combination of measurement levels is required for the use of the phi coefficient?
- What is the specific formula for calculating the phi coefficient?
- Which formula describes the relationship between φ and χ2?
- What is the formula for testing the difference between two independent correlation coefficients?
- What is the rule of thumb for effect size r2 and r?
Workgroup tips 1
Correlation is NOT causation. It is an association between variables.
Positive correlation = both increase or decrease
Negative = One increases, the other decreases
Pearson’s r; both variables are at an interval level. Formula: ∑ZxZy/n-1
Spearman rho = two ordinal variables (To avoid outlier influence in Pearson’s r) rs = r Important: RANK IT FIRST, then take the z scores
Point Biserial; one dichotomous and one continuous variable rpb = r
Phi is a nominal variable, that only has two levels each aka dichotomous X2 = r
Dichotomous means that the value can only be one of two things. For instance yes/no, male/female, left/right. It is a nominal variable, but where with a simply nominal variable answers can be red/blue/green/yellow, a dichotomous variable could in this case only be red/blue, for instance.
Basically, all of these correlation have the basic formula, which is ∑ZxZy/n-1
R is about sample, ρ is population
Parameter | Population | Sample |
Mean | µ | x̄ |
Probability | P | p |
Standard Deviation | σ | S |
Correlation |
Experimenteel en Correlationeel Onderzoek: Samenvattingen, uittreksels, aantekeningen en oefenvragen - UL
- In deze bundel worden o.a. samenvattingen, oefententamens en collegeaantekeningen gedeeld voor het vak Expirimenteel en Correlationeel Onderzoek voor de opleiding Psychologie, jaar 1, aan de Universiteit Leiden
- Engelstalige studiehulp kan je vinden in de bundel bij het vak Experimental and Correlational Research
- Voor een compleet overzicht van de door JoHo aangeboden samenvattingen & studiehulp en de beschikbare geprinte samenvattingen voor dit vak ga je naar de Samenvattingen Startpagina Psychologie Bachelor 1 - UL op WorldSupporter.org
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