The SimilarityLookup operation returns up to ten products per page that are similar to one or more items specified in the request. This operation is typically used to pique a customer’s interest in buying something similar to what they’ve already ordered.

If you specify more than one item, SimilarityLookup returns the intersection of similar items each item would return separately. Alternatively, you can use the SimilarityType parameter to return the union of items that are similar to any of the specified items. A maximum of ten similar items are returned; the operation does not return additional pages of similar items. If there are more than ten similar items, running the same request can result in different answers because the ten that are included in the response are picked randomly. The results are picked randomly only when you specify multiple items and the results include more than ten similar items.

When you specify multiple items, it is possible for there to be no intersection of similar items. In this case, the operation returns the following error:

<Error>
  <Code>AWS.ECommerceService.NoSimilarities</Code>
  <Message>There are no similar items for this ASIN: B00006WREH.</Message>
</Error>

This result is very often the case if the items belong to different search indices. The error can occur, however, even when the items share the same search index.

Similarity is a measurement of similar items purchased, that is, customers who bought X also bought Y and Z. It is not a measure, for example, of items viewed, that is, customers who viewed X also viewed Y and Z.

Items returned can be filtered by:

Return items that are similar to a list of items.

http://ecs.amazonaws.com/onca/xml?
Service=AWSECommerceService&
AWSAccessKeyId=[AWS Access Key ID]&
Operation=SimilarityLookup&ItemId=ASIN1,ASIN2,ASIN3
    

This request returns the intersection of the similarities for each ASIN. The response to this request is shown in Response to Sample Request.

Return up to ten items that are similar to any of the ASINs specified.

http://ecs.amazonaws.com/onca/xml?
Service=AWSECommerceService&
AWSAccessKeyId=[AWS Access Key ID]&
Operation=SimilarityLookup&ItemId=ASIN1,ASIN2,ASIN3&
SimilarityType=Random
    

This request returns the union of items that are similar to all of the ASINs specified. Only ten items can be returned and those are picked randomly from all of the similar items. Repeating the operation could produce different results.

<Item>
  <ASIN>B0009VX8XQ</ASIN>
  <DetailPageURL>http://www.amazon.com/exec/obidos/redirect?tag=ws%26link_code=xm2%26camp=2025%26creative=165953%26path=http://www.amazon.com/gp/redirect.html%253fASIN=B0009VX8XQ%2526tag=ws%2526lcode=xm2%2526cID=2025%2526ccmID=165953%2526location=/o/ASIN/B0009VX8XQ%25253FAWSAccessKeyId=[AWS Access Key ID]</DetailPageURL>
  <ItemAttributes>
    <ProductGroup>Apparel</ProductGroup>
    <Title>Mark VII Men's Short Sleeve Herringbone Knit Golf Shirt</Title>
  </ItemAttributes>
</Item>
<Item>
  <ASIN>B0009VX8VI</ASIN>
  <DetailPageURL>http://www.amazon.com/exec/obidos/redirect?tag=ws%26link_code=xm2%26camp=2025%26creative=165953%26path=http://www.amazon.com/gp/redirect.html%253fASIN=B0009VX8VI%2526tag=ws%2526lcode=xm2%2526cID=2025%2526ccmID=165953%2526location=/o/ASIN/B0009VX8VI%25253FAWSAccessKeyId=[AWS Access Key ID]</DetailPageURL>
  <ItemAttributes>
    <ProductGroup>Apparel</ProductGroup>
    <Title>Mark VII Men's Short Sleeve Knit Golf Shirt</Title>
  </ItemAttributes>
</Item>
  

ItemSearch using the Similarities response group