Put the spike in a different iteration

When possible, it works well to put the investigative story in one iteration and the other stories in one or more subsequent iterations. Normally, only the investigative story can be estimated. Including the other, non-estimatable stories in the same iteration with the investigative story means there will be a higher than normal level of uncertainty about how much can be accomplished in that iteration.

Cohn, Mike (2004-03-01). User Stories Applied: For Agile Software Development (Kindle Locations 921-923). Pearson Education (USA). Kindle Edition. 

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