Classical test theory (CTT and item response theory (IRT) are widely perceived as representing two very different measurement frameworks. However, few studies have empirically examined the similarities and differences in the

parameters estimated using the two frameworks.

Although CTT has served the measurement community for most of this century, IRT has witnessed an exponential growth in recent decades. The major advantage of CTT are its relatively weak theoretical assumptions, which make CTT easy to apply in many testing situations. Relatively weak theoretical assumptions not only characterize CTT but also its extensions (e.g., generalizability theory). Although CTT’s major focus is on test-level information, item statistics (i.e., item difficulty and item discrimination) are also an important part of the CTT model.

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http://www2.hawaii.edu/~daniel/irtctt.pdf

i have two researches in this point, i think the only disadvantage when using IRT is the sample size