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Lecture 11 - Item Response Theory, Most important differences between 1PL…

- - - - What determines the probability of answering an item correct? (1) Difficulty (b) and (2) level of ability of a person
      - Bell curve = the higher the theta on the x-axis the more difficult the questions
      - About the b parameter: When the b parameter is changed: the curve will shift to the left or right. If the item is easier then the line will shift to the left + the b parameter tells us where on the theta dimension the item discriminates most, and how well it discriminates. Easier items discriminate at a lower level
      - IRT is mostly used for cognitive traits, but it can also be sued for measuring fx OCD (=higher level of OCD => the further to the right the line will be)
      - You can think of discrimination as: Does a slight increase/decrease in theta lead to any noticeable differences in the probabilities of answering yes?
  - - - b tells me where the line is steepest, a parameter tells us how steep the line is
        
        If the a parameter = 0, then the items does not discriminate at all => answering the item correct is 50/50
        
        What is equivalent of a? = factor loadings from the common factor model
        
        Remember that a differs across items. So 5 items will have 10 unique parameters (5 b parameters, and 5 a parameters)