Denver teachers have gone on strike to improve their pay, but the fight isn't that simple.

Teachers are challenging one of the nation's oldest incentive pay systems, saying the system is complicated and can leave them guessing at what their earnings will be.

The Denver system allows teachers to add on to their base salary by earning bonuses of up to $3,000 a year for working in a hard-to-staff position or high poverty school or if their schools improve.

Some teachers have problems with systems that base raises on student or teacher performance.

University of North Carolina, Chapel Hill, professor Matthew Springer says there has been a big increase in schools, districts and states experimenting with incentive pay programs nationwide.