The shape of a dataset is hugely important to how well it can be handled by different software. The shape defines how it is laid out: wide as in a spreadsheet, or long as in a database table. Each has its use, but it’s important to understand their differences and when each is the right choice.

Wide and Two-Dimensional: The Spreadsheet

Spreadsheets are laid out in two dimensions. That’s probably their most fundamental and most important feature. They make sense to us because we can easily think in terms of a grid that organizes things in a logical manner.

The grid layout is efficient because it means you don’t have to repeat what is being shown. In this example of unemployment data, each row is a year and each column a month. If you’re looking for, say, April 2010, you know where to look. And you can read time across or down, depending on whether you’re looking to compare between months or year over year.

Calculations also present themselves: sum the rows across (not in this particular example), sum the columns down, get the average, create a new column with the difference of each row from the overall mean, etc.

There are limitations, of course. If you want to break the data down in more than two ways, you need to create multiple tables. The same is true when adding more numbers in each of the cells. You might want to compare unemployment to labor force or education, but they’re in different tables, making this complicated.

For data analysis tools, spreadsheets also pose the challenge that their formats can vary widely and are often inconsistent in subtle but problematic ways. One issue is that there is usually stuff around the actual data to explain things, making it hard for a program to even just reliably figure out what the actual data is.

Another problem is that column headings don’t tell you what they mean. In the example above, Year means that each of the numbers in that column is a year. But Jan, Feb, etc. don’t, they are actually part of the date, and thus part of the data. How is a program supposed to tell them apart?

There are many other conventions in spreadsheets that make sense to people – of course the column in this table is the same as in the one above, of course that single label applies to all rows, etc. –, but are impossible for a machine to figure out.

Long and Skinny: The Database

The opposite of the spreadsheet is the database table. Instead of laying out the data in two dimensions, it’s a long list. It has columns, but not nearly as many (for the same dataset).

Instead, each column has a role. In our unemployment rate example, there is one column for the year, one for the month, and then a final column for the actual value. This is not a good format for a human, but machines love it.

This format causes repetition. The first row represents January 2006, so it has to contain the year 2006 and then the month January. The next entry, February 2006, has to repeat the year. And a year later, the month repeats again. It seems like a lot of effort to represent data this way, but this way it’s all specified. And most databases today can compress this kind of repetitive data so it takes up very little space.

The uniform structure of this format makes it easy to perform all sorts of operations, like filtering out certain values, calculating differences, averages, etc., and all the other things databases are good at.

This format is also much more flexible when it comes to adding more ways of breaking down the data, or adding more measurements. Do you want to have an unemployment rate per state? In the spreadsheet model, you have to make many tables, one for each state. Or you can make larger tables for each state by month, and different tables for each year. This might seem odd, but it would allow comparison between states over each year.

In the long-and-skinny format, you just add a state column and let the machine worry about it. Draw a chart with a line per state over time? Calculate year-over-year change for each? Etc.

Columns that break up the data like that are called dimensions. Database tables often contain dozens of them. The numbers associated with each combination of dimension values are called measures. And those are also easy to add. Want the labor force in addition to the unemployment rate? Just add a column. Same for population, cost of living, etc.

Having all that data in the same place makes for many possible comparisons that are up to the person asking questions, and don’t need to be prepared as tables beforehand. That’s why it’s much better for data to be machine-friendly than human-readable: a machine can turn the machine-friendly data into all sorts of human-readable formats, but the opposite is much more difficult and error-prone.

Turn One Into The Other: (Un-)Pivot

The two shapes look different and they’re useful in different contexts. But they represent exactly the same data. Each can also be transformed into the other. Though whether that is easy or not depends on the tools at hand and some specifics of the data.

The term for turning the long-skinny database format into the wide spreadsheet format is usually to pivot the data. This operation is pretty easy because the machine knows where to find the data, and just needs to be told which dimensions to use to make columns and which measures to include.

The other operation, the unpivot, is much more difficult and error-prone. When the data is already in proper table format (with all the stuff around the pure data removed), it works well though. Tools like Trifacta Wrangler and Tableau can perform unpivots (though Tableau calls it pivot, just to be different).

No Right Shape

Neither of these two data shapes is right or wrong. They each work well for their respective uses. The difficulty is that when data prepared with one use in mind is to be utilized for the other.

More fundamentally, it’s crucial to know that these differences in data shape exist. That’s the first step in trying to figure out why a particular dataset is so damn hard to work with in the tool of your choice. You won’t be very happy with a wide dataset in Tableau, and neither with a long-and-skinny one in Excel.