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Lesson plan Grade 8 Math

Fallow Creek Nutrient Runoff: Scatter Plots and Rainfall Data

A grade 8 lesson where students plot fictional rainfall and nitrate data from a creek, then describe the pattern of association and identify an outlier.

Objectives

  1. Students will construct a scatter plot from bivariate rainfall and nitrate concentration data.
  2. Students will describe the pattern of association shown in a scatter plot, including clustering and an outlier.
  3. Students will use a scatter plot's pattern to make a prediction about a new data point.

Vocabulary

Key words: scatter plot, association, outlier, clustering

Teacher overview

Fallow Creek (a fictional agricultural creek used for this lesson) runs past several farm fields, and its nitrate level shifts with the amount of rain that has recently fallen, since rain washes fertilizer off nearby fields and into the water. This lesson has students plot eight months of fictional rainfall and nitrate readings, describe the pattern the points form, and notice one month that breaks the pattern. A common misconception is describing a scatter plot's association only by whether it "goes up" or "goes down" without also noting how closely the points cluster around that trend, which is exactly why one clear outlier matters for the discussion in this lesson.

Procedure

Hook (5 min): Show two simple sketches on the board: one where dots trend clearly upward and one where dots are scattered with no visible pattern. Ask students which one shows a "relationship" between the two quantities and why.

Direct instruction (15 min): Introduce the fictional Fallow Creek data table (see Student Handout) showing monthly rainfall in inches and nitrate concentration in mg/L. Model plotting the first three points on a grid with rainfall on the horizontal axis and nitrate on the vertical axis, labeling both axes with units. Point out that as rainfall increases from month to month, the nitrate readings generally increase too, which is called a positive association.

Guided practice (15 min): In pairs, students plot the remaining five points from the data table onto their own grid. Circulate and check that axes are labeled and points are placed correctly, especially the point for month 8, which does not follow the rising pattern of the other seven months.

Independent practice (10 min): Students write two or three sentences describing the overall pattern (positive association, roughly linear) and identify which point is the outlier, along with one possible real-world reason a month could break the pattern (for example, a sampling or lab error, or a period when no fertilizer had recently been applied).

Closure / exit ticket (5 min): Students answer one question: "Ignoring the outlier, if next month had 9 inches of rainfall, would you predict the nitrate reading to be higher or lower than any reading so far? Explain in one sentence using the pattern you found."

Fallow Creek Nutrient Runoff: Scatter Plots and Rainfall Data — Grade 8 Math
Name: ____________________________ Date: ______________

Student Handout

Fallow Creek Nutrient Data (fictional data used for this lesson)

Month Rainfall (in) Nitrate (mg/L)
1 1 2
2 2 3
3 3 5
4 4 6
5 5 8
6 6 9
7 7 11
8 8 7

Plot rainfall on the horizontal axis and nitrate concentration on the vertical axis. Label both axes with their units before placing any points.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key
  1. Answer: positive association.
    Independent practice, overall pattern. As rainfall increases across months 1 through 7, nitrate concentration also increases, showing a positive, roughly linear association.
  2. Answer: month 8.
    Independent practice, outlier. Month 8's nitrate reading of 7 mg/L is lower than month 7's reading of 11 mg/L despite having more rainfall, breaking the rising pattern the other seven points follow.
  3. Answer: higher.
    Exit-ticket question. Following the pattern of the other seven months (ignoring the outlier), a higher rainfall total than any seen so far would be expected to produce a higher nitrate reading than any seen so far, since nitrate rises with rainfall in this data set.

Differentiation

Support

Provide a grid with the axes already labeled and scaled, and have students plot only the first four points independently while the teacher plots the remaining four together as a class.

Extension

Ask students to calculate the change in nitrate concentration per inch of rainfall between each consecutive pair of "normal" months (months 1 through 7) and determine whether that rate stays roughly constant, which would suggest the relationship is close to linear rather than just generally positive.

ELL supports

Pre-teach "association," "outlier," and "cluster" with a simple visual: a tight group of dots circled and labeled "cluster," and one dot far from the group circled and labeled "outlier." Allow the independent-practice description to be written as short phrases with an arrow diagram instead of full sentences.