Office of Employee Performance and Evaluation

PGCPS Teacher Effectiveness: “Z - Score” Explained

The Prince George’s County’s Teacher evaluation rating system uses a standardized “Z-Score” to compare teachers across the District.  This concept is predicated on a distribution of effectiveness scores in a ‘normal’ distribution.


Figure 2.  Normal Distribution Curve

 Z Score


Teachers with evaluation scores to the far right of the distribution would be deemed “Highly Effective,” teachers with evaluation scores to the far left would be deemed “Ineffective,” while the majority of the scores that typically fall within the central range would be deemed “Effective.”

The total evaluation score of a teacher is computed as follows:

1.   Each teacher receives scores for the areas of “Professional Practice” and “Student Growth,” as shown on the Teacher Evaluation Model (Figure 1). Each of the two areas is comprised of multiple measures agreed upon by participants and stakeholders, including Administrators, central office staff, and our union partners.

2.   The maximum score attainable is 100 points.

3.   Teachers’ scores are ordered on this 100 point scale.  It is expected that the count of

teachers’ scores will form a graph similar to that shown above.

4.   To set the cut point score for determining whether a teacher is ineffective, effective, or highly effective, statistical methods are employed that reflect historical data and local expectations:

a.   How many (% of) teachers typically receive an “unsatisfactory” rating (using the

      Danielson FfT rating for teacher rubric for teacher observation)?

b.   How many (% of) teachers are really “distinguished” instructors (using the Danielson

      FfT rating for teacher rubric for teacher observation)?

c.   How many (% of) teachers can the system afford to replace each year?

d.   What can be done to raise the performance level of struggling teachers?

A “Z-Score” is determined as follows:

1.     Find the average/mean score (all teachers in PGCPS).

2.     Find the standard deviation (s.d.) of all the scores.

3.     Subtract the average from each score to determine values.

a.    Scores above average will be positive.

b.   Scores below average will be negative.

c.   The average score will be zero.

4.     Divide the values by the standard deviation.

5.     The final values generated represent each teacher’s “Z-Score.”

6.     “Z-Score” values will typically vary within a range of -5 and +5.

7.     In a normal distribution, approximately 6.5% of teachers will have a value below -1.5, approximately 6.5% of teachers will have a value above +1.5, and the remaining 87% will fall within the average range on the distribution curve.


No “Z-Score” can/will be calculated until all teachers’ evaluation scores are available in the system. When we are confident in the actual raw scores that identify levels of effectiveness, specific cut scores will then be established.

Thomas Claggett
Teacher Leadership Center
2001 Addison Road S
District Heights,  Maryland 20747

301-952-6240 phone
301-952-6199 fax 

Last modified: 1/5/2016 2:51:33 PM