(Reuters) - The latest round of store sales data tends to distort the movements of retail stocks, possibly due to a tendency for sales to be discounted in early January when retailers move their items to seasonal merchandise, according to Morgan Stanley.

Thursday’s full-year numbers released by Macy’s Inc and Kohl’s Corp are the most recent example of this, while Friday’s data is likely to reveal similar distortions, analysts said.

“Sales-on-shelf (SOS) data typically rises as POS (store sales) falls in the fourth quarter, to account for the likely reduction in inventory as more new items hit retail outlets,” Morgan Stanley analysts said in a note.

“But we suspect the impact of downsizing in late December and early January is much bigger than usual this year,” Morgan Stanley said.

Macy’s and Kohl’s on Thursday said they would not include the impact of sales-on-shelf or SOS data in their holiday results as the two metrics do not accurately reflect sales trends.

SOS data as it has been presented can be misleading, as it focuses on the full-price sales portion of the transaction while masking the effect of discounting, they said.

The department store operators’ plans to omit the data suggests the change has taken place in the last few years, the analysts said.

Thursday’s robust retail data is likely to be dragged down by weak mid-tier department store chain J.C. Penney Co Inc, which will release its holiday sales report on Friday, they said.

Shares of J.C. Penney fell 1.5 percent in premarket trading on Friday.