Informal Prose Inventories (also known as Informal Reading Inventories) provide valuable insights into the decoding and comprehension strategies that a reader is using. This information, alongside the informal day to day observations you make, will inform decision-making about the needs of your individual learners and help you to design programmes that build on your reader's strengths and address their weakness. 

PART 1 OF THE INFORMAL PROSE INVENTORY TEST:

Oral Reading of the test passage - Assessing Decoding Strategies

Listening to and analysing a student's oral reading behaviour is still the most effective method of observing and understanding the decoding strategies that a reader is using. 

Running Record procedure (Marie Clay)

The Accuracy score that you get from the first oral reading of the IPI test gives you an overall picture of how the reader is coping with this level of text difficulty.  An accuracy score below 94% indicates that the level of difficulty is too high - the decoding task will be absorbing too much of the reader's working memory for them to be able to process the meaning. A score of 97%+ indicates a recreational level - the reader is not experiencing any significant word recognition or decoding difficulties. A score between 94% and 97% indicates that this is the correct level of difficulty for your reading instruction.
Miscue Analysis procedure (Ken Goodman)

The Miscue Analysis - analysing the decoding behaviour - gives additional information about the cues that the reader is using to decode the text. The evidence of a 'fluent decoder' is that they cross-check, self-correct, and self-monitor using all three cuing sources of information, visual, meaning, and syntax, to solve decoding problems.

PART 2 OF THE INFORMAL PROSE INVENTORY TEST:

Silent Reading of the test passage followed by Retelling and Questioning

Measuring the use of comprehension strategies is not so straightforward as these occur 'in the head' of the reader and are not evident to the observer. As a result the IPI test can only measure the level of comprehension or understanding of the passage, which will by default indicate whether comprehension strategies have been employed or not.

Retelling as a measure of comprehension

Retelling gives you useful information about the reader's ability to reconstruct the text; are they remembering random facts with little attention to sequence, or have they identified the structure of the text and used this to hang the information on. A 50% retell of all the significant detail represents a pass at each level of difficulty. While giving insight into the use of text structure (an important comprehension strategy) this is primarily a measure of comprehension, not of the use of the text structure comprehension strategy.
Answering questions as a measure of comprehension

Answering explicit literal questions straight from the text quickly establishes whether the face value content of the passage has been understood. Each passage also includes 2 inferential questions to give you an indication of the reader's ability to "read between the lines". Once again, questioning does not measure the use of comprehension strategies, only the level of comprehension. 

The need for Comprehension Strategy Instruction

A common result is to find that while a student is reading a piece of text with a high degree of accuracy (97-100%) in the inital phase of the test, their comprehension scores are below the established criteria (Retell 50%, Response to questions 75%).
This suggests that the reader has well developed word recognition and decoding skills but is a passive reader - not interacting with, or processing the text - and highlights the need for Comprehension Strategy Instruction as outlined in this website.

What Comprehension Strategy Instruction is not
A traditional view of developing comprehension skills is to require the reader to answer lots of questions, either in oral discussion or in a written format after reading. It is important to realise that questioning only tests comprehension. It does nothing to develop strategies. In some cases it will stimulate thinking which results in deeper understanding. However the reader is still dependant on the questions to unpack the text. Comprehension Strategy Instruction advocates the teaching of metacognitive strategies so that the reader knows how to unpack the text themselves. 

More on Comprehension Strategy Instruction ...