A researcher investigates the effect of job training on wages. Especially, a researcher wants to know whether there is any difference in the effect of job training on wages by college major. There are three majors in the data engineering, humanity, and social science. A researcher tried to estimate the equation (1) but it fails to get all coefficients.

ln()=0+1 +2 +3 +4 +, (1)

where is a job training indicator, , , and are the interaction terms between job training and college majors.

However, a research can estimate all coefficients in the equation (2).

ln()=0+1 +2 +3 + (2)

Why this research could not get all coefficients the equation (1) but can get all coefficients in the equation (2)? Explain it.