diff --git a/lib/period-length.js b/lib/period-length.js
new file mode 100644
index 0000000..f369571
--- /dev/null
+++ b/lib/period-length.js
@@ -0,0 +1,46 @@
+import assert from 'assert'
+
+export default function getPeriodLengthStats(cycleLengths) {
+  throwIfArgsAreNotInRequiredFormat(cycleLengths)
+  const periodLengthStats = {}
+  const sortedCycleLengths = cycleLengths.sort((a, b) => {
+    return a - b
+  })
+  periodLengthStats.minimum = sortedCycleLengths[0]
+  periodLengthStats.maximum = sortedCycleLengths[cycleLengths.length - 1]
+  periodLengthStats.mean = Math.round(
+    cycleLengths.reduce(getSum) / cycleLengths.length * 100) / 100
+  // median
+  if (cycleLengths.length % 2 == 1) {
+    periodLengthStats.median = sortedCycleLengths[(cycleLengths.length + 1) /
+       2 - 1]
+  }
+  else {
+    const middle = cycleLengths.length / 2
+    periodLengthStats.median = (sortedCycleLengths[middle - 1] +
+       sortedCycleLengths[middle]) / 2
+  }
+  // corrected standard deviation (based on unbiased sample variance)
+  periodLengthStats.stdDeviation = null // for case t
+  if (cycleLengths.length > 1) {
+    const sumOfSquares = cycleLengths.map(cycleLength => {
+      return Math.pow(cycleLength - periodLengthStats.mean, 2)
+    }).reduce(getSum)
+    periodLengthStats.stdDeviation = Math.round(
+      Math.sqrt(sumOfSquares / (cycleLengths.length - 1 )) * 100) / 100
+  }
+  return periodLengthStats
+}
+
+function getSum(total, num) {
+  return total + num
+}
+
+function throwIfArgsAreNotInRequiredFormat(cycleLengths) {
+  assert.ok(Array.isArray(cycleLengths), 'Function requires input to be an array.')
+  assert.ok(cycleLengths.length > 0, 'Input array should not be empty.')
+  cycleLengths.forEach(cycleLength => {
+    assert.equal(typeof cycleLength, 'number', 'Elements in the array should be of type number.')
+    assert.ok(!isNaN(cycleLength), 'Elements of Array should not be NaN.')
+  })
+}
\ No newline at end of file
diff --git a/lib/periode-length.js b/lib/periode-length.js
deleted file mode 100644
index 2445685..0000000
--- a/lib/periode-length.js
+++ /dev/null
@@ -1,35 +0,0 @@
-export default function getPeriodLengthStats(cycleLengthArray) {
-  if (Array.isArray(cycleLengthArray) && (cycleLengthArray.length > 0) && cycleLengthArray.every(cycleLength => {
-    return (typeof cycleLength === 'number') && !(isNaN(cycleLength))
-  })) {
-    const PeriodLengthStats = {}
-    const sortedCycleLegthArray = cycleLengthArray.sort((a, b) => {
-      return a - b
-    })
-    PeriodLengthStats.minimum = sortedCycleLegthArray[0]
-    PeriodLengthStats.maximum = sortedCycleLegthArray[cycleLengthArray.length - 1]
-    PeriodLengthStats.mean = cycleLengthArray.reduce(getSum) / cycleLengthArray.length
-    // median
-    if (cycleLengthArray.length % 2 == 1) {
-      PeriodLengthStats.median = sortedCycleLegthArray[(cycleLengthArray.length + 1) / 2 - 1]
-    }
-    else {
-      const middle = cycleLengthArray.length / 2
-      PeriodLengthStats.median = (sortedCycleLegthArray[middle - 1] + sortedCycleLegthArray[middle]) / 2
-    }
-
-    PeriodLengthStats.stdDeviation = 0 // default
-    if (cycleLengthArray.length > 1) {
-      const sumOfSquares = cycleLengthArray.map(cycleLength => {
-        return Math.pow(cycleLength - PeriodLengthStats.mean, 2)
-      }).reduce(getSum)
-      PeriodLengthStats.stdDeviation = Math.sqrt(sumOfSquares / (cycleLengthArray.length - 1 ))
-    }
-    return PeriodLengthStats
-  }
-  console.error('getPeriodLengthStats requiers an array of numbers with length > 0.')
-}
-
-function getSum(total, num) {
-  return total + num
-}
\ No newline at end of file
diff --git a/test/periode-length.spec.js b/test/periode-length.spec.js
index 3051244..ad10bd6 100644
--- a/test/periode-length.spec.js
+++ b/test/periode-length.spec.js
@@ -1,16 +1,52 @@
 import chai from 'chai'
+import { AssertionError } from 'assert'
+
 import periodInfo from '../lib/period-length'
 
 const expect = chai.expect
 
-describe('it calculates the median correctly', () => {
-  it('works for an odd-numbered array', () => {
-    const periodLengths = [1, 2, 5, 99, 100]
-    const result = periodInfo(periodLengths).median
-    expect(result).to.eql(5)
+describe('getPeriodLengthStats', () => {
+  it('works for a simple odd-numbered array', () => {
+    const periodLengths = [99, 5, 1, 2, 100]
+    const result = periodInfo(periodLengths)
+    const expectedResult = {
+      minimum: 1,
+      maximum: 100,
+      mean: 41.4,
+      median: 5,
+      stdDeviation: 53.06
+    }
+    expect(result).to.eql(expectedResult)
   })
 
-  /* it('works for an even-numbered array', () => {
-
-  }) */
+  it('works for a simple even-numbered array', () => {
+    const periodLengths = [4, 1, 15, 2, 20, 5]
+    const result = periodInfo(periodLengths)
+    const expectedResult = {
+      minimum: 1,
+      maximum: 20,
+      mean: 7.83,
+      median: 4.5,
+      stdDeviation: 7.78
+    }
+    expect(result).to.eql(expectedResult)
+  })
+  describe('when args are wrong', () => {
+    it('throws when arg object is an empty array', () => {
+      const periodLengths = []
+      expect(() => periodInfo(periodLengths).to.throw(AssertionError))
+    })
+    it('throws when arg object is not in right format', () => {
+      const wrongObject = { hello: 'world' }
+      expect(() => periodInfo(wrongObject).to.throw(AssertionError))
+    })
+    it('throws when arg array contains a string', () => {
+      const wrongElement = [4, 1, 15, '2', 20, 5]
+      expect(() => periodInfo(wrongElement).to.throw(AssertionError))
+    })
+    it('throws when arg array contains a NaN', () => {
+      const wrongElement = [4, 1, 15, NaN, 20, 5]
+      expect(() => periodInfo(wrongElement).to.throw(AssertionError))
+    })
+  })
 })
\ No newline at end of file