Autore della sezione: Danielle J. Navarro and David R. Foxcroft
The spreadsheet
It is possible to simply begin typing values into the jamovi spreadsheet as you would in any other spreadsheet software. Alternatively, existing data sets can be opened in jamovi (see the section “Loading data in jamovi” further down on the page).
In jamovi data is represented in a spreadsheet with each column representing a “variable” and each row representing a “case” or “participant”.
Data Variables
The most commonly used variables in jamovi are Data
variables, these
variables simply contain data either loaded from a data file, or “typed in”
by the user. Data
variables can be one of four measurement levels.
These levels are designated by the symbol in the header of the
variable’s column:
The
ID
variable type is unique to jamovi. It’s intended for variables that contain identifiers that you would almost never want to analyse. For example, a persons name, or a participant ID. Specifying an ID variable type can improve performance when interacting with very large data sets.Nominal
variables are for categorical variables which are text labels, for example a column called Gender with the valuesMale
andFemale
would be nominal. So would a person’s name. Nominal variable values can also have a numeric value. These variables are used most often when importing data which codes values with numbers rather than text. For example, a column in a dataset may contain the values 1 forMale
, and 2 forFemale
. It is possible to add nice “human-readable” labels to these values with the variable editor (more on this later).Ordinal
variables are likeNominal
variables, except the values have a specific order. An example is a Likert scale with 3 being “strongly agree” and -3 being “strongly disagree”.Continuous
variables are variables which exist on a continuous scale. Examples might be height or weight. This is also referred to as “interval scale” or “ratio scale”.
In addition, you can also specify different data types: variables have a
data type of either Text
, Integer
or Decimal
.
When starting with a blank spreadsheet and typing values in the variable type will change automatically depending on the data you enter. This isa good way to get a feel for which variable types go with which sorts of data. Similarly, when opening a data file jamovi will try and guess the variable type from the data in each column. In both cases this automatic approach may not be correct, and it may be necessary to manually specify the variable type with the variable editor.
The variable editor can be opened by selecting Setup
from the Data
ribbon or by double-clicking on the variable column header. The variable
editor allows you to change the name of the variable and, for data variables,
the measure type, the order of the value levels, and the label displayed for
each level. The variable editor can be dismissed by clicking ↑
.
New variables can be inserted or appended to the data set using the Add
button from the Data
tab. The Add
button also allows the addition
of computed variables.
Sometimes you want to change the variable level. This can happen for all sorts of reasons. Sometimes when you import data from files, it can come to you in the wrong format. Numbers sometimes get imported as nominal , text values. Dates may get imported as text. Participant-ID values can sometimes be read as continuous : nominal values can sometimes be read as ordinal or even continuous . There’s a good chance that sometimes you’ll want to convert a variable from one measurement level into another one. Or, to use the correct term, you want to coerce the variable from one class into another.
If you want to change a variable’s measurement level then you can do this in
the jamovi Data
view. Click on the variable name in the top row of the data
table and then select the desired measurement level under Measure Type
–
continuous , ordinal or nominal .
Computed variables
Computed Variables are those which take their value by performing a computation on other variables. Computed Variables can be used for a range of purposes, including log transforms, z-scores, sum-scores, negative scoring and means. There is another variable type, Transformed variables, that can be used to “recode” variables (e.g., when inverting items). This variable type is briefly described at the end of the subsection EFA in jamovi and in Fig. 180.
Computed variables can be added to the data set with the Add
button
available on the data tab. This will produce a formula box where you can
specify the formula. The usual arithmetic operators are available. Some
examples of formulas are:
A + B
LOG10(len)
MEAN(A, B)
(dose - VMEAN(dose)) / VSTDEV(dose)
In order, these are the sum of A
and B
, a log (base 10) transform
of len
, the mean of A
and B
, and the z-score of the variable
dose
. Fig. 4 shows the jamovi screen for the
new variable computed as the z-score of dose
(from the Tooth Growth
example data set).
V-functions
Several functions are already available in jamovi and available from the
drop down box labelled fx. A number of functions appear in pairs,
one prefixed with a V and the other not. V functions perform their
calculation on a variable as a whole, where as non-V functions perform
their calculation row by row. For example, MEAN(A, B)
will produce the
mean of A
and B
for each row. Where as VMEAN(A)
gives the mean of
all the values in A
.
Loading data in jamovi
There are several different types of files that are likely to be relevant to us when doing data analysis. There are two in particular that are especially important from the perspective of this book:
jamovi files are those with a
.omv
file extension. This is the standard kind of file that jamovi uses to store data, and variables and analyses.Comma separated value (CSV) files are those with a
.csv
file extension. These are just regular old text files and they can be opened with many different software programs. It’s quite typical for people to store data in CSV files, precisely because they’re so simple.
There are also several other kinds of data file that you might want to
import into jamovi. For instance, you might want to open Microsoft Excel
spreadsheets (.xlsx
files), or data files that have been saved in the
native file formats for other statistics software, such as SPSS or SAS.
Whichever file formats you are using, it’s a good idea to create a folder or folders especially for your jamovi data sets and analyses and to make sure you keep these backed up regularly.
To open a file select the main jamovi menu (☰
; top left hand corner),
select Open
and then choose from the files listed under This PC
if you
want to open an file stored on your computer or select an example data set by
choosing Data Library
. The example files in this book can be found within
the Data Library
→ learning statistics with jamovi
(or lsj-data
).
Importing data from CSV files
One quite commonly used data format is the humble “comma separated value”
file, also called a CSV file, and usually bearing the file extension .csv
.
CSV files are just plain old-fashioned text files and what they store is
basically just a table of data. This is illustrated in
Fig. 5, which shows a file called booksales.csv
that I’ve
created. As you can see, each row represents the book sales data for one
month. The first row doesn’t contain actual data though, it has the names of
the variables.
It’s easy to open CSV files in jamovi. From the jamovi main menu (☰
; top
left hand corner) choose Open
and browse to where you have stored the CSV
file on your computer. If you’re on a Mac, it’ll look like the usual Finder
window that you use to choose a file; on Windows it looks like an Explorer
window. I’m assuming that you’re familiar with your own computer, so you
should have no problem finding the CSV-file that you want to import! Find the
one you want, then click on the Open
button.
There are a few things that you can check to make sure that the data gets imported correctly:
Heading. Does the first row of the file contain the names for each variable - a “header” row? The
booksales.csv
file has a header, so that’s a yes.Separator. What character is used to separate different entries? In most CSV files this will be a comma (it is “comma separated” after all).
Decimal. What character is used to specify the decimal point? In English speaking countries this is almost always a period (i.e.,
.
). That’s not universally true though, many European countries use a comma.Quote. What character is used to denote a block of text? That’s usually going to be a double quote mark (
"
). It is for thebooksales.csv
file.
Throughout this book I’ve assumed that your data are stored as a jamovi
.omv
file or as a “properly” formatted CSV file. However, in real
life that’s not a terribly plausible assumption to make so I’d better
talk about some of the other possibilities that you might run into.
The first thing I should point out is that if your data are saved as a text file but aren’t quite in the proper CSV format then there’s still a pretty good chance that jamovi will be able to open it. You just need to try it and see if it works. Sometimes though you will need to change some of the formatting. The ones that I’ve often found myself needing to change are:
header
. A lot of the time when you’re storing data as a CSV file the first row actually contains the column names and not data. If that’s not true then it’s a good idea to open up the CSV file in a spreadsheet programme such as LibreOffice and add the header row manually.sep
. As the name “comma separated value” indicates, the values in a row of a CSV file are usually separated by commas. This isn’t universal, however. In Europe the decimal point is typically written as,
instead of.
and as a consequence it would be somewhat awkward to use,
as the separator. Therefore it is not unusual to use;
instead of,
as the separator. At other times, I’ve seen a TAB character used.quote
. It’s conventional in CSV files to include a quoting character for textual data. As you can see by looking at thebooksales.csv
file, this is usually a double quote character,"
. But sometimes there is no quoting character at all, or you might see a single quote mark’
used instead.skip
. It’s actually very common to receive CSV files in which the first few rows have nothing to do with the actual data. Instead, they provide a human readable summary of where the data came from, or maybe they include some technical info that doesn’t relate to the data.missing values
. Often you’ll get given data with missing values. For one reason or another, some entries in the table are missing. The data file needs to include a “special” value to indicate that the entry is missing. By default jamovi assumes that this value isNA
,[1] for both numeric and text data, so you should make sure that, where necessary, all missing values in the CSV file are replaced with99
(or-9999
; whichever you choose) before opening / importing the file into jamovi. Once you have opened / imported the file into jamovi all the missing values are converted to blank or greyed out cells in the jamovi spreadsheet view. You can also change the missing value for each variable as an option in theData
→Setup
view.
Importing data from other statistics packages
The commands listed above are the main ones we’ll need for data files in
this book. But in real life we have many more possibilities. For
example, you might want to read data files in from other statistics
programs. Since SPSS is probably the most widely used statistics package
in psychology, it’s worth mentioning that jamovi can also import SPSS
data files (file extension .sav
). Just follow the instructions above
for how to open a CSV file, but this time navigate to the .sav
file
you want to import.
As far as other statistical software goes, jamovi can also directly open / import a wealth of other formats such as R, SAS, STATA, Excel, LibreOffice, and JSON.