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Dichotomous Key

8th Grade Performance Task
Contributed by: New York State Education Department (NYSED)
NYS Alternative Assessment in Science Project (1996)


This activity is to used as an assessment after the students have already been exposed to the use of a dichotomous key, and therefore know what the term dichotomous refers to, and how it is used. There are 13 species of Triangulum pictured; it is suggested that each student identifies any three of the species.

  • 1 point is awarded for each correct choice made in the identification of each species.
  • 1 point is awarded for each correct choice or step along the way.

    The scoring of each species should continue until the last correct choice or step.

    The Example given on the Student Answer Sheet (#101) would be awarded 3 points for a perfect answer. But if the student had only been able to reach choice 1B before making a mistake, only 1 point would be awarded.

Since a different number of choices is necessary to identify the different species, the students may end up with different total points. Therefore a percent grade should be given.

Total number of points for each species are given below, including the correct choice for each of the steps.

Species Number

Species Name

Correct Steps

Possible Points

Species # 101

T. waveus (10B)


3 points

Species # 102

T. monoflagelleum (6A)


5 points

Species # 103

T. roundflagelleum (12B)


6 points

Species # 104

T. polycilius (9B)


5 points

Species # 105

T. pointiatus (8A)


6 points

Species # 106

T. polyflagelleum (6B)


5 points

Species # 107

T. ventridotteus (11A)


4 points

Species # 108

T. equalius (4A)


5 points

Species # 109

T. biciliatus (9A)


5 points

Species # 110

T. oddcilius (7A)


5 points

Species # 111

T. roundiatus (8B)


6 points

Species # 112

T. pointiflagelleum (12A)


6 points

Species # 113

T. dorsalidotteus (11B)


4 points

Total Number of Points - 65 points

Sample Answer Sheet

Choose any of the three species of Triangulum from the species sheet and key them to their scientific names. Be sure to write in the numbers of the species that you are trying to identify. In the proper spaces below write in the number and letter (example #101) for each of the choices you made as you identified the species. When you are sure of the species identification, write in the scientific name in the space provided.

Total actual points - 9

Total possible points - 16

Total actual points / Total possible points X 100 = 56 percent grade

Scoring Form - Dichotomous Key

Student ID # ____________________

Female / Male (circle one)



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