What is SignLogic?
SignLogic is a set of options to explicitly tell the Tagger whether to report a number as positive or negative. You may need to change this depending on the context of your reporting or if a value is being used in a calculation. When you get an Invalid Calculation error, or your calculation doesn’t add up correctly, you will need to check the SignLogic applied to the cells involved in the calculation.
Please be aware Invalid Calculation errors are only Warnings. This type of error does not have an impact on your final report and, subject to taking advice from your auditors, can be disregarded or the calculations removed.
Contents
What is SignLogic?
Why do we need to use SignLogic?
How do I pick the correct SignLogic for a cell?
SignLogic Examples
SignLogic Options
Additional Videos and Reading
Why do we need to use SignLogic?
In a financial report, all of these numbers mean the same thing:
 5123
 (5123)
 We reported a loss of £5,123
If you, a human, were asked to subtract 5,123 from 8,000 you wouldn’t care about how the number was written or how the instruction was given. As a human, you would naturally understand that 8000  5123 is 2877. Additionally, you understand that, even though there is an implicit positive sign in the sentence "We reported a loss of £5,123", the context means that this is actually a negative number.
These nuances become extremely important when we create a calculation in the Tagger. The calculation result will be wrong if the Tagger isn’t handling positive/negative numbers in the way that you expect. Reporting software must be told explicitly how to handle these signs (hence the name SignLogic). Reporting software could potentially understand the instruction to “subtract 5,123 from 8,000” to mean 8000  5123. The two negative signs would cancel each other out and this subtraction would become an addition with the result 13,123! Additionally, there is no way for software to understand any implicit contexts  we must explicitly tell the software how to handle positive and negative signs.
There are two questions we need to ask when we add a number with an XBRL tag to a calculation :

 Is the tag a credit or a debit type (i.e. is the fact itself positive or negative)?;
 Is the reported number positive or negative?
The answers to these two questions will have an impact on how the value of the number is understood by software and tell us whether we need to change the SignLogic of the cell. For example, consider a debit tag applied to a negative number. The two negatives would cancel each other out and the number would be reported as a positive (which is the result in the Debit/Debit cell below). We would need to change the SignLogic of the cell to get the correct result.
The table below shows the results of applying credit/debit weights to positive and negative values:
How do I pick the correct SignLogic for a cell?
The following four rules will give you a good understanding of how to approach picking the correct SignLogic for each cell in your report. Please be aware that, although each cell has a SignLogic value, the default value of Always Positive is usually correct (see Rule #3 below). Following these rules are some examples and a table that explains all the possible outputs of the different SignLogic options on credit and debit tags with positive and negative numbers.
Rule #1  It doesn’t matter how the number is written in your report
The SignLogic option you need to pick may very well be different from how the number is written in the report. It will always depend on how the number is intended to be reported and the specific circumstance of the reporting. For example, in the sentence “We reported a loss of £5,123” the number itself is written as a positive but is intended to be understood as a negative number.
Rule #2  You cannot change the weight of a tag
When you add tagged cells to a calculation you will see that each tag has a weight of either +1 or 1. This weight shows whether the balance of the tag is credit or debit and applies the corresponding +/ sign to the reported value. The correct weight/sign will be automatically applied and it cannot be changed. To change how the Tagger is interpreting a number you must change the SignLogic. You can do this in the Table Cell Properties tab.
Rule #3  Different SignLogic options mean different things
There are five options for SignLogic:
 None: no special sign logic will be applied.
 As Reported: the value will always be added to the iXBRL document with the same sign as the sign in the report.
 Always Positive: the value will always be reported with a positive sign.
 Always Negative: the value will always be reported with a negative sign.
 Reverse: the sign will be reversed. Negative values will be reported as positive values and positive values as negative values.
The default value for SignLogic in XBRL is Always Positive. This is because, in most cases, this is the correct choice. Applying a positive sign to a positive number has no impact at all, and applying a positive sign to a negative number ensures it stays a negative number.
Rule #4  There’s two easy formulae that makes all this make sense
 parsed value = shown value with SignLogic applied
 calculated value = parsed value x weight
You can see all of these values for the calculations in your report by clicking Options/Calculation Report from the Tagger top menu bar. This will create an Excel file with columns for Shown Value, Parsed Value, SignLogic and Calculated Value. If you have multiple tables in your file each table will be shown in its own tab. The examples below show how these formulae work.
Additionally, see below for a full list of the outcomes of different SignLogic options on the different balance types with positive and negative numbers.
Example 1  Understanding these formulae
Let’s assume we have:
 A value written in our report of 10;
 With a SignLogic value of None;
 And a credit (+1) tag applied (e.g. Revenue).
If we run out a Calculation Report for this table ( by clicking Options/Calculation Report from the Tagger top menu bar) we will see the following:
Shown value  SignLogic  Parsed value  Weight  Calculation value 

10  None  10  +1  10 
First we need to understand our parsed value. Our shown value in the report is 10 and there is no special logic being applied so our parsed value is 10. Next we need to multiply our parsed value by the weight. In this case that’s 10 multiplied by +1, so the value that will be used in our calculation will be +10. This is correct as it matches the shown value so we don't need to make any changes to this cell's SignLogic.
Example 2  Changing a value with SignLogic
This time, let’s assume we have:
 A value written in our report of 10;
 With a SignLogic value of None;
 And a debit (1) tag applied (e.g. Cost of Sales).
If we run out a Calculation Report for this table ( by clicking Options/Calculation Report from the Tagger top menu bar) we will see the following:
Shown value  SignLogic  Parsed value  Weight  Calculation value 

10  None  10  1  10 
First we need to understand our parsed value. Our shown value in the report is 10 and there is no special logic being applied so our parsed value is 10. Next we need to multiply our parsed value by the weight. In this case that’s 10 multiplied by 1, so the value that will be used in our calculation will be +10. This is exactly what we want to avoid  a negative number is being treated as a positive number in the calculation  and we will need to change the SignLogic of the cell to get the correct result.
If we change the SignLogic to Always Positive (by selecting the cell and changing the SignLogic in the Table Cell Properties tab) we will get the correct result:
Shown value  SignLogic  Parsed value  Weight  Calculation value 

10  Always Positive  10  1  10 
First we need to understand our parsed value. Our shown value in the report is 10 but this time the Always Positive SignLogic is being applied. This means that the value will always be reported with a positive sign and so our parsed value is +10.
Next we need to multiply our parsed value by the weight. In this case that’s +10 multiplied by 1, so the value that will be used in our calculation will be 10. This number is now behaving as expected.
See below for a full list of the outcomes of different SignLogic options on the different balance types with positive and negative numbers to help you make the right choice.
SignLogic options
The table below shows all the possible outcomes of positive and negative values, positive and negative weights and SignLogics. As such, some of these combinations are not ones that you would want to use in your report  they are included for illustrative purposes only. Use this to work out which SignLogic option you need to use:
You can run out your own version of this table for your report by clicking Options/Calculation Report from the Tagger top menu bar. This will create an Excel file with columns for Shown Value, Parsed Value, SignLogic and Calculated Value where it’s easy to see if a number has the wrong sign. If you have multiple tables in your file each table will be shown in its own tab.
Additional Reading
Calculations and Signs video tutorial – (web.ctrlprint.net)
Reporting negative values for IFRS taxonomy elements – section 5 – (ifrs.org)