A class of 20 students answered "What is your favourite pet?" The answers, in the order they were given, are: Dog, Cat, Dog, Fish, Cat, Dog, Bird, Cat, Dog, Fish, Dog, Cat, Bird, Dog, Cat, Fish, Dog, Cat, Bird, Dog. Step 1: Work through the list in order. For each answer, make ONE tally mark beside its category, and cross the answer off so it cannot be counted twice. Step 2: Count the tallies. Dog: one group of five and three single marks = 8. Cat: one group of five and one single mark = 6. Fish: 3 single marks = 3. Bird: 3 single marks = 3. Step 3: Write the frequency table: Dog 8, Cat 6, Fish 3, Bird 3, Total 20. Step 4: CHECK. Add the frequencies: 8 + 6 + 3 + 3 = 20. There were 20 answers, so every answer has been counted exactly once. The frequencies must always add up to the number of data points. If they do not, an answer was missed or counted twice. Say it plainly: a tally mark is a promise - one mark for one answer, never two, never zero.

Sixteen students chose a favourite season: Spring, Summer, Summer, Fall, Winter, Summer, Spring, Fall, Summer, Winter, Summer, Fall, Spring, Summer, Fall, Winter. Build the frequency table and check the total. -> Spring 3, Summer 6, Fall 4, Winter 3; total 3 + 6 + 4 + 3 = 16