Productivity and difficulty
Productivity refers to metrics and measures of output from production processes, per unit of input. Productivity P may be conceived of as a metrics of the technical or engineering efficiency of production. In software project planning , productivity is defined as the number of lines of code developed per person-month
Difficulty The ratio (K/td2), where K is software development cost and td is peak development time, is known as difficulty and shown by D, which is measured in person/year.