diff --git a/Sprint-1/fix/median.js b/Sprint-1/fix/median.js
index b22590bc6..5c5b796e1 100644
--- a/Sprint-1/fix/median.js
+++ b/Sprint-1/fix/median.js
@@ -2,8 +2,8 @@
 // Start by running the tests for this function
 // If you're in the Sprint-1 directory, you can run `npm test -- fix` to run the tests in the fix directory
 
-// Hint: Please consider scenarios when 'list' doesn't have numbers (the function is expected to return null)
-// or 'list' has mixed values (the function is expected to sort only numbers).
+// Hint: Please consider scenarios when 'list' isn't an array, is empty,
+// or contains values that aren't numbers (the function is expected to throw - see the tests).
 
 function calculateMedian(list) {
   const middleIndex = Math.floor(list.length / 2);
diff --git a/Sprint-1/fix/median.test.js b/Sprint-1/fix/median.test.js
index 21da654d7..51d61cc86 100644
--- a/Sprint-1/fix/median.test.js
+++ b/Sprint-1/fix/median.test.js
@@ -7,25 +7,45 @@
 const calculateMedian = require("./median.js");
 
 describe("calculateMedian", () => {
-  [
-    { input: [1, 2, 3], expected: 2 },
-    { input: [1, 2, 3, 4, 5], expected: 3 },
-    { input: [1, 2, 3, 4], expected: 2.5 },
-    { input: [1, 2, 3, 4, 5, 6], expected: 3.5 },
-  ].forEach(({ input, expected }) =>
-    it(`returns the median for [${input}]`, () => expect(calculateMedian(input)).toEqual(expected))
-  );
-
-  [
-    { input: [3, 1, 2], expected: 2 },
-    { input: [5, 1, 3, 4, 2], expected: 3 },
-    { input: [4, 2, 1, 3], expected: 2.5 },
-    { input: [6, 1, 5, 3, 2, 4], expected: 3.5 },
-    { input: [110, 20, 0], expected: 20 },
-    { input: [6, -2, 2, 12, 14], expected: 6 },
-  ].forEach(({ input, expected }) =>
-    it(`returns the correct median for unsorted array [${input}]`, () => expect(calculateMedian(input)).toEqual(expected))
-  );
+  it("returns the median for [1, 2, 3]", () => {
+    expect(calculateMedian([1, 2, 3])).toEqual(2);
+  });
+
+  it("returns the median for [1, 2, 3, 4, 5]", () => {
+    expect(calculateMedian([1, 2, 3, 4, 5])).toEqual(3);
+  });
+
+  it("returns the median for [1, 2, 3, 4]", () => {
+    expect(calculateMedian([1, 2, 3, 4])).toEqual(2.5);
+  });
+
+  it("returns the median for [1, 2, 3, 4, 5, 6]", () => {
+    expect(calculateMedian([1, 2, 3, 4, 5, 6])).toEqual(3.5);
+  });
+
+  it("returns the correct median for unsorted array [3, 1, 2]", () => {
+    expect(calculateMedian([3, 1, 2])).toEqual(2);
+  });
+
+  it("returns the correct median for unsorted array [5, 1, 3, 4, 2]", () => {
+    expect(calculateMedian([5, 1, 3, 4, 2])).toEqual(3);
+  });
+
+  it("returns the correct median for unsorted array [4, 2, 1, 3]", () => {
+    expect(calculateMedian([4, 2, 1, 3])).toEqual(2.5);
+  });
+
+  it("returns the correct median for unsorted array [6, 1, 5, 3, 2, 4]", () => {
+    expect(calculateMedian([6, 1, 5, 3, 2, 4])).toEqual(3.5);
+  });
+
+  it("returns the correct median for unsorted array [110, 20, 0]", () => {
+    expect(calculateMedian([110, 20, 0])).toEqual(20);
+  });
+
+  it("returns the correct median for unsorted array [6, -2, 2, 12, 14]", () => {
+    expect(calculateMedian([6, -2, 2, 12, 14])).toEqual(6);
+  });
 
   it("doesn't modify the input array [3, 1, 2]", () => {
     const list = [3, 1, 2];
@@ -33,18 +53,66 @@ describe("calculateMedian", () => {
     expect(list).toEqual([3, 1, 2]);
   });
 
-  [ 'not an array', 123, null, undefined, {}, [], ["apple", null, undefined] ].forEach(val =>
-    it(`returns null for non-numeric array (${val})`, () => expect(calculateMedian(val)).toBe(null))
-  );
-
-  [
-    { input: [1, 2, "3", null, undefined, 4], expected: 2 },
-    { input: ["apple", 1, 2, 3, "banana", 4], expected: 2.5 },
-    { input: [1, "2", 3, "4", 5], expected: 3 },
-    { input: [1, "apple", 2, null, 3, undefined, 4], expected: 2.5 },
-    { input: [3, "apple", 1, null, 2, undefined, 4], expected: 2.5 },
-    { input: ["banana", 5, 3, "apple", 1, 4, 2], expected: 3 },
-  ].forEach(({ input, expected }) =>
-    it(`filters out non-numeric values and calculates the median for [${input}]`, () => expect(calculateMedian(input)).toEqual(expected))
-  );
+  // There is no median of an empty array, so calculateMedian should throw
+  it("throws when given an empty array", () => {
+    expect(() => calculateMedian([])).toThrow(
+      new Error("calculateMedian requires a non-empty array")
+    );
+  });
+
+  // Input that isn't an array should throw
+  it("throws when given a string", () => {
+    expect(() => calculateMedian("banana")).toThrow(
+      new Error("calculateMedian requires an array of numbers")
+    );
+  });
+
+  it("throws when given a number", () => {
+    expect(() => calculateMedian(123)).toThrow(
+      new Error("calculateMedian requires an array of numbers")
+    );
+  });
+
+  it("throws when given null", () => {
+    expect(() => calculateMedian(null)).toThrow(
+      new Error("calculateMedian requires an array of numbers")
+    );
+  });
+
+  it("throws when given an object", () => {
+    expect(() => calculateMedian({})).toThrow(
+      new Error("calculateMedian requires an array of numbers")
+    );
+  });
+
+  it("throws when called with no argument", () => {
+    expect(() => calculateMedian()).toThrow(
+      new Error("calculateMedian requires an array of numbers")
+    );
+  });
+
+  // Arrays containing any non-number value should throw, rather than filtering them out
+  it("throws for an array of strings", () => {
+    expect(() => calculateMedian(["ten", "twenty", "thirty"])).toThrow(
+      new Error("calculateMedian requires an array of numbers")
+    );
+  });
+
+  it("throws for an array mixing numbers and strings", () => {
+    expect(() => calculateMedian([1, "2", 3, "4", 5])).toThrow(
+      new Error("calculateMedian requires an array of numbers")
+    );
+  });
+
+  it("throws for an array containing null", () => {
+    expect(() => calculateMedian([1, 2, null, 4])).toThrow(
+      new Error("calculateMedian requires an array of numbers")
+    );
+  });
+
+  it("throws for an array containing undefined", () => {
+    expect(() => calculateMedian([3, 1, undefined, 2])).toThrow(
+      new Error("calculateMedian requires an array of numbers")
+    );
+  });
 });
diff --git a/Sprint-1/implement/describe-median.js b/Sprint-1/implement/describe-median.js
new file mode 100644
index 000000000..3381cfc0a
--- /dev/null
+++ b/Sprint-1/implement/describe-median.js
@@ -0,0 +1,25 @@
+// Don't change this function.
+function calculateMedian(list) {
+  if (!Array.isArray(list)) {
+    throw new Error("calculateMedian requires an array of numbers");
+  }
+  if (list.length === 0) {
+    throw new Error("calculateMedian requires a non-empty array");
+  }
+  for (const item of list) {
+    if (typeof item !== "number") {
+      throw new Error("calculateMedian requires an array of numbers");
+    }
+  }
+  const middleIndex = Math.floor(list.length / 2);
+  const sorted = [...list].sort((a, b) => a - b);
+  if (sorted.length % 2 === 0) {
+    return (sorted[middleIndex - 1] + sorted[middleIndex]) / 2;
+  }
+  return sorted[middleIndex];
+}
+
+// Implement this function. See describe-median.test.js for the acceptance criteria.
+function describeMedian(list) {}
+
+module.exports = describeMedian;
diff --git a/Sprint-1/implement/describe-median.test.js b/Sprint-1/implement/describe-median.test.js
new file mode 100644
index 000000000..e7d18a264
--- /dev/null
+++ b/Sprint-1/implement/describe-median.test.js
@@ -0,0 +1,32 @@
+/* Describe the median, or explain why there isn't one
+
+calculateMedian is given to you at the top of describe-median.js. It throws
+when it can't calculate a median. Don't change it.
+
+Implement describeMedian, which calls calculateMedian and returns a sentence
+instead of crashing.
+
+E.g. describeMedian([1, 2, 3]), target output: "The median is 2"
+E.g. describeMedian([]), target output: "Could not calculate a median: calculateMedian requires a non-empty array"
+
+You'll need a try/catch block for this. The catch block must use the error's
+message, so the caller can see what went wrong.
+*/
+
+const describeMedian = require("./describe-median.js");
+
+// Acceptance criteria:
+
+// Given an array of numbers
+// When passed to describeMedian
+// Then it should return "The median is " followed by the median
+// Delete this test.todo and replace it with a test.
+test.todo('given [1, 2, 3], returns "The median is 2"');
+
+// Given an empty array
+// When passed to describeMedian
+// Then it should return "Could not calculate a median: calculateMedian requires a non-empty array"
+
+// Given something that isn't an array of numbers, e.g. "banana"
+// When passed to describeMedian
+// Then it should return "Could not calculate a median: calculateMedian requires an array of numbers"
diff --git a/Sprint-1/implement/max.test.js b/Sprint-1/implement/max.test.js
index 82f18fd88..ff3fbb5e5 100644
--- a/Sprint-1/implement/max.test.js
+++ b/Sprint-1/implement/max.test.js
@@ -3,7 +3,7 @@
 In this kata, you will need to implement a function that find the largest numerical element of an array.
 
 E.g. max([30, 50, 10, 40]), target output: 50
-E.g. max(['hey', 10, 'hi', 60, 10]), target output: 60 (sum ignores any non-numerical elements)
+E.g. max(['hey', 10, 'hi', 60, 10]) throws Error("findMax requires an array of numbers") (max can't compare non-numerical elements, so it shouldn't guess)
 
 You should implement this function in max.js, and add tests for it in this file.
 
@@ -34,10 +34,10 @@ test.todo("given an empty array, returns -Infinity");
 // When passed to the max function
 // Then it should return the largest decimal number
 
-// Given an array with non-number values
+// Given an array containing a value that isn't a number
 // When passed to the max function
-// Then it should return the max and ignore non-numeric values
+// Then it should throw Error("findMax requires an array of numbers")
 
-// Given an array with only non-number values
+// Given something that isn't an array at all, such as "hey", 42 or no argument
 // When passed to the max function
-// Then it should return the least surprising value given how it behaves for all other inputs
+// Then it should throw Error("findMax requires an array of numbers")
diff --git a/Sprint-1/implement/mean.js b/Sprint-1/implement/mean.js
new file mode 100644
index 000000000..36909ea1f
--- /dev/null
+++ b/Sprint-1/implement/mean.js
@@ -0,0 +1,3 @@
+function calculateMean(list) {}
+
+module.exports = calculateMean;
diff --git a/Sprint-1/implement/mean.test.js b/Sprint-1/implement/mean.test.js
new file mode 100644
index 000000000..847b88676
--- /dev/null
+++ b/Sprint-1/implement/mean.test.js
@@ -0,0 +1,50 @@
+/* Calculate the mean of an array of numbers
+
+In this kata, you will need to implement a function that calculates the mean of an array of numbers.
+
+E.g. calculateMean([1, 2, 6]), target output: 3
+E.g. calculateMean([]) throws Error("calculateMean requires a non-empty array")
+E.g. calculateMean("banana") throws Error("calculateMean requires an array of numbers")
+
+There is no mean of an empty array, and a string isn't an array of numbers, so
+calculateMean should throw rather than return a value. This is the same problem
+calculateMedian solves in the prep and in the fix directory.
+
+You should implement this function in mean.js, and add tests for it in this file.
+
+We have set things up already so that this file can see your function from the other file.
+*/
+
+const calculateMean = require("./mean.js");
+
+// Acceptance criteria:
+
+// Given an array of numbers
+// When passed to calculateMean
+// Then it should return their mean
+// Delete this test.todo and replace it with a test.
+test.todo("given [1, 2, 6], returns 3");
+
+// Given an array with a single number
+// When passed to calculateMean
+// Then it should return that number
+
+// Given an array containing negative or decimal numbers
+// When passed to calculateMean
+// Then it should return the correct mean
+
+// Given an empty array
+// When passed to calculateMean
+// Then it should throw Error("calculateMean requires a non-empty array")
+
+// Given a value that isn't an array, e.g. "banana", 42, null or {}
+// When passed to calculateMean
+// Then it should throw Error("calculateMean requires an array of numbers")
+
+// Given no argument at all
+// When passed to calculateMean
+// Then it should throw Error("calculateMean requires an array of numbers")
+
+// Given an array containing a non-number value, e.g. [1, "2", 3]
+// When passed to calculateMean
+// Then it should throw Error("calculateMean requires an array of numbers")
diff --git a/Sprint-1/implement/sum.test.js b/Sprint-1/implement/sum.test.js
index dd0a090ca..a288e0f0a 100644
--- a/Sprint-1/implement/sum.test.js
+++ b/Sprint-1/implement/sum.test.js
@@ -3,7 +3,7 @@
 In this kata, you will need to implement a function that sums the numerical elements of an array
 
 E.g. sum([10, 20, 30]), target output: 60
-E.g. sum(['hey', 10, 'hi', 60, 10]), target output: 80 (ignore any non-numerical elements)
+E.g. sum(['hey', 10, 'hi', 60, 10]) throws Error("sum requires an array of numbers") (sum can't add non-numerical elements, so it shouldn't guess)
 */
 
 const sum = require("./sum.js");
@@ -27,10 +27,10 @@ test.todo("given an empty array, returns 0")
 // When passed to the sum function
 // Then it should return the correct total sum
 
-// Given an array containing non-number values
+// Given an array containing a value that isn't a number
 // When passed to the sum function
-// Then it should ignore the non-numerical values and return the sum of the numerical elements
+// Then it should throw Error("sum requires an array of numbers")
 
-// Given an array with only non-number values
+// Given something that isn't an array at all, such as "hey", 42 or no argument
 // When passed to the sum function
-// Then it should return the least surprising value given how it behaves for all other inputs
+// Then it should throw Error("sum requires an array of numbers")
diff --git a/Sprint-1/readme.md b/Sprint-1/readme.md
index cf282a64b..8a35c3405 100644
--- a/Sprint-1/readme.md
+++ b/Sprint-1/readme.md
@@ -31,6 +31,8 @@ Here is a recommended order:
 1. `max.test.js`
 2. `sum.test.js`
 3. `dedupe.test.js`
+4. `mean.test.js`
+5. `describe-median.test.js`
 
 Again, you can run just the test for the `implement` directory if you're in the `Sprint-1` directory by running `npm test -- implement`.