Use Mathematical Expressions to Transform your Variables

Modified on Fri, 19 Aug, 2022 at 1:54 PM

When performing statistical tests, you can transform your variables by using mathematical expressions.

One common example is to use the logarithm of a variable. Let's say you are studying a biological parameter with a very large spread from 10 to 1.000.000. Instead of using the raw value, you might want to analyze the logarithm of this value to reduce the range to 1 to 6 instead of 10 to 1.000.000.

How to use transformations?

1. Click on the cog wheel next to the name of the variable:

2. Then, head to the transformation menu:

Use a logarithmic transformation

Logarithmic transformations are available from the transformation menu as described above.

Just pick the logarithmic transformation you want. Then validate by clicking on the "Confirm" button.

Beware! If you have 0 or negative values, the logarithmic transformations will not be applied and an error message will be raised.

Other mathematical expressions

A wide range of mathematical expressions is available to transform your variables.

To use a custom mathematical expression:

Choose "Custom" in the transformation menu
Type a formula using "x" for the variable, for example: "x + 100" to add 100 to your variable

Operators

Expression	Definition	Comment / Example
+	Plus	x + 100
-	Minus	100 - x
*	Multiply
/	Divide	If the variable data contains 0, an error will be raised and the formula will not be applied
** or ^ or pow	Power	x ** 2 is x squared
%	Modulo
==	Is equal to
!=	Is not equal to
>	Is greater than
<	Is lower than
>=	Is greater or equal
<=	Is lower or equal
and	And	To be used with the "if" expression
if(a, b, c)	If "a" then "b" else "c"	if(x>=100 and x<=200, x, 100) If the value of the variable is between 100 and 200, then we use the variable. Else, we define the value as 100.
in(a, b, ...)	Belongs to	if(x in (1, 2, 5), 0, 1) If the value of the variable is 1, 2 or 5, then we define the value as 0, else we defined the value as 1
not	Not	if(not(x in (1, 2, 5), 0, 1) This expression will have the opposite behaviour of the previous example
or	Or	if(x==1 or x==2, 0, 1) If x is 1 or 2, then we define the value as 0, else we define the value as 1
xor	Exclusive Or	https://en.wikipedia.org/wiki/Exclusive_or

Functions

Expression	Definition	Comment / Example
abs(x)	Absolute value
acos(x)	Arc cosinus, x in radians
acosd(x)	Arc cosinus, x in degrees
asin(x)	Arc sinus, x in radians
asind(x)	Arc sinus, x in degrees
atan(x)	Arc tangent, x in radians
atand(x)	Arc tangent, x in degrees
atan2(x)	Two-arguments arc tangent
ceil(x)	Upper-value rounding	Rounds to the closest integer greater than x ceil(2.3) = 3
cos(x)	Cosinus, x in radians
cosd(x)	Cosinus, x in degrees
exp(x)	Exponential
exp(x, base)	Exponential in a specific base
fac(x)	Factorial of x
floor(x)	Lower-value rounding	Rounds to the closest integer lower than x floor(2.3) = 2
log(x)	Logarithm
log(x, base)	Logarithm in a specific base
max(x1, x2, ..., xn)	Maximum value between x1, x2, ...	max(x, 100) will always return a value greater or equal to 100
min(x1, x2, ..., xn)	Minimum value between x1, x2, ...	min(x, 100) will always return a value lower or equal to 100
pyt(x, y)	Pythagorian value of x,y: sqrt(x^2 + y^2)
random(x)	Random number in [0, x]
round(x)	Integer rounding	Rounds to the closest integer round(2.3) = 2 round(2.7) = 3
round(x, n)	Rounding to n digits
sin(x)	Sinus, x in radians
sind(x)	Sinus, x in degrees
sqrt(x)	Squared root of x
tan(x)	Tangent, x in radians
tand(x)	Tangent, x in degrees

Constant values

Constant	Definition	Comment
E	Euleur's number	~ 2.71828
PI	Pi π	~ 3.141592