differences - Interdependence vs. interdependency: grammatical number

Are the two words interchangeable? If so, why is one more common in the singular (interdependence) and the other in the plural (interdependencies)?

# interdependence > # interdependency
# interdependencies > # interdependences

Look at the Google search hit numbers below:

[Edit: Updated the numbers 10-10-18. Note that Google just estimates — based on what it finds in the first moments of its search — the probable number of search results for a given term. But the crude approximation is good enough for this purpose.]

interdependence: 21,200,000
interdependences: 160,000

interdependency: 12,100,000
interdependencies: 3,920,000

Answer

Whereas, elementary/primary school teachers would propagate to little children as countable/non-countable nouns, I wish to advise that engineering would prefer the more mathematical terms of

• qualitative vs quantitative


• categories vs values

There are four possibilities

1. XXX-ence and XXX-ency are used interchangeably, with one usage being more prominent than the other.
   e.g., Existence vs existency.




2. XXX-ence is used both qualitatively and quantitatively, where XXX-ency is not valid.
   e.g., reference where there is "no such thing as" referency.




3. XXX-ence is qualitative.
   XXX-ency is quantitative.
   e.g., dependence vs dependency




4. XXX-ence is qualitative.
   XXX-ency is quantitative.
   but XXX-ency is often used in place of XXX-ence.
   e.g., dependence vs dependency

In engineering, we would normally say,

We need to prove there is dependence between the two systems. And then we need to discover and map out those dependencies. And then we should resolve the indirection (i.e. remove the ambiguity of direction) of those dependencies.

Since the words interdependence(y) are derived from dependence(y), then

• interdependence is qualitative


• interdependency is quantitative

e.g.,

Could you scour your repertoire of expertise to find statistical methods we could use to discover any interdependence between these parameters, and if so, using the same repertoire of methods to correlate the interdependencies that exist between them?

In engineering, we would use dependencies in place of dependence. Because the quantitative idea would logically imply the qualitative idea. But not the converse, as we would never say,

Let us discover all the dependences

because it sounds tacky, perhaps.