Summary Statistics

Summary statistics help people quickly understand datasets and make informed decisions. Many interesting datasets are stored in spreadsheet files.

SheetJS is a JavaScript library for reading and writing data from spreadsheets.

This demo uses SheetJS to process data in spreadsheets. We'll explore how to extract spreadsheet data and how to compute simple summary statistics. This demo will focus on two general data representations:

• "Arrays of Objects" simplifies processing by translating from the SheetJS data model to a more idiomatic data structure.
• "Dense Worksheets" directly analyzes SheetJS worksheets.

The Import Tutorial is a guided example of extracting data from a workbook. It is strongly recommended to review the tutorial first.

Tested Deployments

This browser demo was tested in the following environments:

Browser	Date
Chrome 126	2024-06-21
Safari 17.4	2024-06-20

Data Representations

Many worksheets include one header row followed by a number of data rows. Each row is an "observation" and each column is a "variable".

The "Array of Objects" explanations use more idiomatic JavaScript patterns. It is suitable for smaller datasets.

The "Dense Worksheets" approach is more performant, but the code patterns are reminiscent of C. The low-level approach is only encouraged when the traditional patterns are prohibitively slow.

Arrays of Objects

The idiomatic JavaScript representation of the dataset is an array of objects. Variable names are typically taken from the first row. Those names are used as keys in each observation.

Spreadsheet	JS Data
	[ { Name: "Bill Clinton", Index: 42 }, { Name: "GeorgeW Bush", Index: 43 }, { Name: "Barack Obama", Index: 44 }, { Name: "Donald Trump", Index: 45 }, { Name: "Joseph Biden", Index: 46 } ]

The SheetJS sheet_to_json method¹ can generate arrays of objects from a worksheet object. For example, the following snippet fetches a test file and creates an array of arrays from the first sheet:

const url = "https://docs.sheetjs.com/typedarray/iris.xlsx";

/* fetch file and pull file data into an ArrayBuffer */
const file = await (await fetch(url)).arrayBuffer();

/* parse workbook */
const workbook = XLSX.read(file, {dense: true});

/* first worksheet */
const first_sheet = workbook.Sheets[workbook.SheetNames[0]];

/* generate array of arrays */
const aoo = XLSX.utils.sheet_to_json(first_sheet);

Dense Worksheets

SheetJS "dense" worksheets² store cells in an array of arrays. The SheetJS read method³ accepts a special dense option to create dense worksheets.

The following example fetches a file:

/* fetch file and pull file data into an ArrayBuffer */
const url = "https://docs.sheetjs.com/typedarray/iris.xlsx";
const file = await (await fetch(url)).arrayBuffer();

/* parse workbook */
const workbook = XLSX.read(file, {dense: true});

/* first worksheet */
const first_dense_sheet = workbook.Sheets[workbook.SheetNames[0]];

The "!data" property of a dense worksheet is an array of arrays of cell objects⁴. Cell objects include attributes including data type and value.

Analyzing Variables

Individual variables can be extracted by looping through the array of objects and accessing specific keys. For example, using the Iris dataset:

Array of Objects

The following snippet shows the first entry in the array of objects:

{
  "sepal length": 5.1,
  "sepal width": 3.5,
  "petal length": 1.4,
  "petal width": 0.2,
  "class ": "Iris-setosa"
}

The values for the sepal length variable can be extracted by indexing each object. The following snippet prints the sepal lengths:

for(let i = 0; i < aoo.length; ++i) {
  const row = aoo[i];
  const sepal_length = row["sepal length"];
  console.log(sepal_length);
}

Result

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Live Editor

function SheetJSAoOExtractColumn() {
  const [col, setCol] = React.useState([]);

  React.useEffect(() => { (async() => {
    const ab = await (await fetch("/typedarray/iris.xlsx")).arrayBuffer();
    const wb = XLSX.read(ab, {dense: true});
    const aoo = XLSX.utils.sheet_to_json(wb.Sheets[wb.SheetNames[0]]);
    /* store first 5 sepal lengths in an array */
    const col = [];
    for(let i = 0; i < aoo.length; ++i) {
      const row = aoo[i];
      const sepal_length = row["sepal length"];
      col.push(sepal_length); if(col.length >= 5) break;
    }
    setCol(col);
  })(); }, []);

  return ( <>
  <b>First 5 Sepal Length Values</b><br/>
  <table><tbody><tr>{col.map(sw => (<td>{sw}</td>))}</tr></tbody></table>
  </>
  );
}

Dense Worksheet

The column for the sepal length variable can be determined by testing the cell values in the first row.

Finding the column index for the variable

The first row of cells will be the first row in the "!data" array:

const first_row = first_dense_sheet["!data"][0];

When looping over the cells in the first row, the cell must be tested in the following order:

• confirm the cell object exists (entry is not null)
• cell is a text cell (the t property will be "s"⁵)
• cell value (v property⁶) matches "sepal length"

let C = -1;
for(let i = 0; i < first_row.length; ++i) {
  let cell = first_row[i];
  /* confirm cell exists */
  if(!cell) continue;
  /* confirm cell is a text cell */
  if(cell.t != "s") continue;
  /* compare the text */
  if(cell.v.localeCompare("sepal length") != 0) continue;
  /* save column index */
  C = i; break;
}
/* throw an error if the column cannot be found */
if(C == -1) throw new Error(`"sepal length" column cannot be found! `);

Finding the values for the variable

After finding the column index, the rest of the rows can be scanned. This time, the cell type will be "n"⁷ (numeric). The following snippet prints values:

const number_of_rows = first_dense_sheet["!data"].length;
for(let R = 1; R < number_of_rows; ++R) {
  /* confirm row exists */
  let row = first_dense_sheet["!data"][R];
  if(!row) continue;
  /* confirm cell exists */
  let cell = row[C];
  if(!cell) continue;
  /* confirm cell is a numeric cell */
  if(cell.t != "n") continue;
  /* print raw value */
  console.log(cell.v);
}

Live Demo

The following snippet prints the sepal lengths:

for(let i = 0; i < aoo.length; ++i) {
  const row = aoo[i];
  const sepal_length = row["sepal length"];
  console.log(sepal_length);
}

Result

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Live Editor

function SheetJSDensExtractColumn() {
  const [msg, setMsg] = React.useState("");
  const [col, setCol] = React.useState([]);

  React.useEffect(() => { (async() => {
    const ab = await (await fetch("/typedarray/iris.xlsx")).arrayBuffer();
    const wb = XLSX.read(ab, {dense: true});
    /* first worksheet */
    const first_dense_sheet = wb.Sheets[wb.SheetNames[0]];

    /* find column index */
    const first_row = first_dense_sheet["!data"][0];
    let C = -1;
    for(let i = 0; i < first_row.length; ++i) {
      let cell = first_row[i];
      /* confirm cell exists */
      if(!cell) continue;
      /* confirm cell is a text cell */
      if(cell.t != "s") continue;
      /* compare the text */
      if(cell.v.localeCompare("sepal length") != 0) continue;
      /* save column index */
      C = i; break;
    }
    /* throw an error if the column cannot be found */
    if(C == -1) return setMsg(`"sepal length" column cannot be found! `);

    /* store first 5 sepal lengths in an array */
    const col = [];
    const number_of_rows = first_dense_sheet["!data"].length;
    for(let R = 1; R < number_of_rows; ++R) {
      /* confirm row exists */
      let row = first_dense_sheet["!data"][R];
      if(!row) continue;
      /* confirm cell exists */
      let cell = row[C];
      if(!cell) continue;
      /* confirm cell is a numeric cell */
      if(cell.t != "n") continue;
      /* add raw value */
      const sepal_length = cell.v;
      col.push(sepal_length); if(col.length >= 5) break;
    }

    setCol(col);
    setMsg("First 5 Sepal Length Values");
  })(); }, []);

  return ( <><b>{msg}</b><br/><table><tbody>
    {col.map(sw => (<tr><td>{sw}</td></tr>))}
  </tbody></table></> );
}

Average (Mean)

For a given sequence of numbers $x_1\mathellipsis x_{count}$ the mean $M$ is defined as the sum of the elements divided by the count:

$$M[x;count] = \frac{1}{count}\sum_{i=1}^{count} x_i$$

In JavaScript terms, the mean of an array is the sum of the numbers in the array divided by the total number of numeric values.

Non-numeric elements and array holes do not affect the sum and do not contribute to the count. Algorithms are expected to explicitly track the count and cannot assume the array length property will be the correct count.

This definition aligns with the spreadsheet AVERAGE function.

AVERAGEA differs from AVERAGE in its treatment of string and Boolean values: string values are treated as zeroes and Boolean values map to their coerced numeric equivalent (true is 1 and false is 0).

JavaScript Ecosystem

Some JavaScript libraries implement functions for computing array means.

Library	Implementation
jStat8	Textbook sum (divide at end)
simple-statistics9	Neumaier compensated sum (divide at end)
stdlib.js10	Trial mean (mean) / van Reeken (incrmean)

Textbook Sum

The mean of a sequence of values can be calculated by computing the sum and dividing by the count.

Array of Objects

The following function accepts an array of objects and a key.

function aoa_average_of_key(aoo, key) {
  let sum = 0, cnt = 0;
  for(let R = 0; R < aoo.length; ++R) {
    const row = aoo[R];
    if(typeof row == "undefined") continue;

    const field = row[key];
    if(typeof field != "number") continue;

    sum += field; ++cnt;
  }
  return cnt == 0 ? 0 : sum / cnt;
}

Live Demo (click to show)

Result

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Live Editor

function SheetJSAoOAverageKey() {
  const [avg, setAvg] = React.useState(NaN);

  function aoa_average_of_key(aoo, key) {
    let sum = 0, cnt = 0;
    for(let R = 0; R < aoo.length; ++R) {
      const row = aoo[R];
      if(typeof row == "undefined") continue;

      const field = row[key];
      if(typeof field != "number") continue;

      sum += field; ++cnt;
    }
    return cnt == 0 ? 0 : sum / cnt;
  }

  React.useEffect(() => { (async() => {
    const ab = await (await fetch("/typedarray/iris.xlsx")).arrayBuffer();
    const wb = XLSX.read(ab, {dense: true});
    const aoo = XLSX.utils.sheet_to_json(wb.Sheets[wb.SheetNames[0]]);
    setAvg(aoa_average_of_key(aoo, "sepal length"));
  })(); }, []);

  return ( <b>The average Sepal Length is {avg}</b> );
}

Dense Worksheet

The following function accepts a SheetJS worksheet and a column index.

function ws_average_of_col(ws, C) {
  const data = ws["!data"];
  let sum = 0, cnt = 0;
  for(let R = 1; R < data.length; ++R) {
    const row = data[R];
    if(typeof row == "undefined") continue;

    const field = row[C];
    if(!field || field.t != "n") continue;

    sum += field.v; ++cnt;
  }
  return cnt == 0 ? 0 : sum / cnt;
}

Live Demo (click to show)

Result

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Live Editor

function SheetJSDenseAverageKey() {
  const [avg, setAvg] = React.useState(NaN);

  function ws_average_of_col(ws, C) {
    const data = ws["!data"];
    let sum = 0, cnt = 0;
    for(let R = 1; R < data.length; ++R) {
      const row = data[R];
      if(typeof row == "undefined") continue;

      const field = row[C];
      if(!field || field.t != "n") continue;

      sum += field.v; ++cnt;
    }
    return cnt == 0 ? 0 : sum / cnt;
  }

  React.useEffect(() => { (async() => {
    const ab = await (await fetch("/typedarray/iris.xlsx")).arrayBuffer();
    const wb = XLSX.read(ab, {dense: true});
    const ws = wb.Sheets[wb.SheetNames[0]];

    /* find column index */
    const first_row = ws["!data"][0];
    let C = -1;
    for(let i = 0; i < first_row.length; ++i) {
      let cell = first_row[i];
      /* confirm cell exists */
      if(!cell) continue;
      /* confirm cell is a text cell */
      if(cell.t != "s") continue;
      /* compare the text */
      if(cell.v.localeCompare("sepal length") != 0) continue;
      /* save column index */
      C = i; break;
    }

    setAvg(ws_average_of_col(ws, C));
  })(); }, []);

  return ( <b>The average Sepal Length is {avg}</b> );
}

The textbook method suffers from numerical issues when many values of similar magnitude are summed. As the number of elements grows, the absolute value of the sum grows to orders of magnitude larger than the absolute values of the individual values and significant figures are lost.

van Reeken

Some of the issues in the textbook approach can be addressed with a differential technique. Instead of computing the whole sum, it is possible to calculate and update an estimate for the mean.

The van Reeken array mean can be implemented in one line of JavaScript code:

for(var n = 1, mean = 0; n <= x.length; ++n) mean += (x[n-1] - mean)/n;

Math details (click to show)

Let $M[x;m] = \frac{1}{m}\sum_{i=1}^{m}x_m$ be the mean of the first $m$ elements. Then:

$M[x;m+1]$	$= \frac{1}{m+1}\sum_{i=1}^{m+1} x_i$
	$= \frac{1}{m+1}\sum_{i=1}^{m} x_i + \frac{x_{m+1}}{m+1}$
	$= \frac{m}{m+1}(\frac{1}{m}\sum_{i=1}^{m} x_i) + \frac{x_{m+1}}{m+1}$
	$= \frac{m}{m+1}M[x;m] + \frac{x_{m+1}}{m+1}$
	$= (1 - \frac{1}{m+1})M[x;m] + \frac{x_{m+1}}{m+1}$
	$= M[x;m] + \frac{x_{m+1}}{m+1} - \frac{1}{m+1}M[x;m]$
	$= M[x;m] + \frac{1}{m+1}(x_{m+1}-M[x;m])$
$new\_mean$	$= old\_mean + (x_{m+1}-old\_mean) / (m+1)$

Switching to zero-based indexing, the relation matches the following expression:

new_mean = old_mean + (x[m] - old_mean) / (m + 1);

This update can be succinctly implemented in JavaScript:

mean += (x[m] - mean) / (m + 1);

Array of Objects

The following function accepts an array of objects and a key.

function aoa_mean_of_key(aoo, key) {
  let mean = 0, cnt = 0;
  for(let R = 0; R < aoo.length; ++R) {
    const row = aoo[R];
    if(typeof row == "undefined") continue;

    const field = row[key];
    if(typeof field != "number") continue;

    mean += (field - mean) / ++cnt;
  }
  return cnt == 0 ? 0 : mean;
}

Live Demo (click to show)

Result

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Live Editor

function SheetJSAoOMeanKey() {
  const [avg, setAvg] = React.useState(NaN);

  function aoa_mean_of_key(aoo, key) {
    let mean = 0, cnt = 0;
    for(let R = 0; R < aoo.length; ++R) {
      const row = aoo[R];
      if(typeof row == "undefined") continue;

      const field = row[key];
      if(typeof field != "number") continue;

      mean += (field - mean) / ++cnt;
    }
    return cnt == 0 ? 0 : mean;
  }

  React.useEffect(() => { (async() => {
    const ab = await (await fetch("/typedarray/iris.xlsx")).arrayBuffer();
    const wb = XLSX.read(ab, {dense: true});
    const aoo = XLSX.utils.sheet_to_json(wb.Sheets[wb.SheetNames[0]]);
    setAvg(aoa_mean_of_key(aoo, "sepal length"));
  })(); }, []);

  return ( <b>The average Sepal Length is {avg}</b> );
}

Dense Worksheet

The following function accepts a SheetJS worksheet and a column index.

function ws_mean_of_col(ws, C) {
  const data = ws["!data"];
  let mean = 0, cnt = 0;
  for(let R = 1; R < data.length; ++R) {
    const row = data[R];
    if(typeof row == "undefined") continue;

    const field = row[C];
    if(!field || field.t != "n") continue;

    mean += (field.v - mean) / ++cnt;
  }
  return cnt == 0 ? 0 : mean;
}

Live Demo (click to show)

Result

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Live Editor

function SheetJSDenseMeanKey() {
  const [avg, setAvg] = React.useState(NaN);

  function ws_mean_of_col(ws, C) {
    const data = ws["!data"];
    let mean = 0, cnt = 0;
    for(let R = 1; R < data.length; ++R) {
      const row = data[R];
      if(typeof row == "undefined") continue;

      const field = row[C];
      if(!field || field.t != "n") continue;

      mean += (field.v - mean) / ++cnt;
    }
    return cnt == 0 ? 0 : mean;
  }

  React.useEffect(() => { (async() => {
    const ab = await (await fetch("/typedarray/iris.xlsx")).arrayBuffer();
    const wb = XLSX.read(ab, {dense: true});
    const ws = wb.Sheets[wb.SheetNames[0]];

    /* find column index */
    const first_row = ws["!data"][0];
    let C = -1;
    for(let i = 0; i < first_row.length; ++i) {
      let cell = first_row[i];
      /* confirm cell exists */
      if(!cell) continue;
      /* confirm cell is a text cell */
      if(cell.t != "s") continue;
      /* compare the text */
      if(cell.v.localeCompare("sepal length") != 0) continue;
      /* save column index */
      C = i; break;
    }

    setAvg(ws_mean_of_col(ws, C));
  })(); }, []);

  return ( <b>The average Sepal Length is {avg}</b> );
}

Historical Context

This algorithm is generally attributed to Welford¹¹. However, the original paper does not propose this algorithm for calculating the mean!

Programmers including Neely¹² attributed a different algorithm to Welford. van Reeken¹³ reported success with the algorithm presented in this section.

Knuth¹⁴ erroneously attributed this implementation of the mean to Welford.

Footnotes

1. See sheet_to_json in "Utilities" ↩

2. See "Dense Mode" in "Utilities" ↩

3. See read in "Reading Files" ↩

4. See "Dense Mode" in "Utilities" ↩

5. See "Cell Types" in "Cell Objects" ↩

6. See "Underlying Values" in "Cell Objects" ↩

7. See "Cell Types" in "Cell Objects" ↩

8. See mean() in the jStat documentation. ↩

9. See mean in the simple-statistics documentation. ↩

10. See incrsum in the stdlib.js documentation. ↩

11. See "Note on a Method for Calculated Corrected Sums of Squares and Products" in Technometrics Vol 4 No 3 (1962 August). ↩

12. See "Comparison of Several Algorithms for Computation of Means, Standard Deviations and Correlation Coefficients" in CACM Vol 9 No 7 (1966 July). ↩

13. See "Dealing with Neely's Algorithms" in CACM Vol 11 No 3 (1968 March). ↩

14. See "The Art of Computer Programming: Seminumerical Algorithms" Third Edition page 232. ↩