The data lead to the assumption that x has no relationship to y.
If you base a conclusion on data, then your conclusion is a deduction, not an assumption. In fact, in experimental science assumptions are usually avoided. A purpose of controls is to eliminate the need to assume anything.
The word ‘data’ is plural.
However since investigators usually refer to sets of data, there is a tendency to use the word as though it was singular. Hence a writer will state, ‘the data was affected by the phase of the moon,’ or ‘the data suggests that phase of the moon has no effect on mood.’ As awkward as it may seem to you, the proper phrases are, ‘the data were affected…,’ and ‘the data suggest…’ By the way, the singular form is ‘datum.’