Over the course of the year, the Trendline covered everything from hot topics like teacher evaluation and salary (including this interactive map) to the nitty gritty of teacher planning time and the length of teacher contracts. As the year draws to a close, here is a brief look at our most popular topics of 2015:

Layoffs: The most read Trendline of 2015

All the way back in January, we took a look at the policies that govern teacher layoffs. Almost half of all districts in the Teacher Contract Database require that non-tenured teachers be laid off before tenured teachers; after that, 42 percent of districts lay off tenured teachers by seniority, 40 percent lay off tenured teachers by performance and 10 percent of districts use multiple measures.

Movement in evaluation policies

Perhaps one of the most talked about policy areas in education, teacher performance evaluations, covered in the March Trendline, proved to be another highly-read topic. Just under 75 percent of districts require that objective student growth data count towards a teacher's evaluation rating, with more districts having plans to include student growth in the future. Forty-one districts counted student growth as 50 percent of a teacher's evaluation in the 2014-2015 school year. 

As the recent November Trendline illustrates, more districts are not only including student growth in teacher evaluations but are also connecting evaluation ratings to other personnel policies. In 2014-2015, 44 percent of districts connected teacher salaries and evaluation in some way, up from only 28 percent of districts in 2012-2013. 

Contract length and health insurance premiums

Two other popular topics this year were length of teacher contracts and health insurance premiums. The average length of collective bargaining agreements in the Teacher Contract Database is 2.82 years, with the most common length being three years. 

Frequently a topic in contract negotiations, health insurance is one important benefit for teachers. Of the districts that specify the maximum portion that the district must pay towards employees' health insurance premiums, about two thirds pay at least 90 percent of the full premium, with many paying the entire premium. 

For complete coverage of these trends, plus all of our other 2015 topics, use the links below. Is there a topic we missed in 2015? Make sure we don't miss it in 2016! Send your ideas for future editions to teachertrendline@nctq.org.

January: Layoffs

February: Health insurance premiums

March: Teacher evaluations

April: Transfers and excessing

May: Substitute teachers

June: Planning and collaboration time

July: Teacher leave

September: Lifetime earnings

October: Length of contracts

November: The role of student achievement and teacher performance in evaluations, compensation and dismissal