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Research materials can be inaccessible for a number of different reasons.
A document only shows up in a database search if it has been "indexed," meaning a record has been created to represent the document in the database. Indexing typically includes the citation information, an abstract or summary, subject headings, the language of publication, and other details that are helpful for evaluating the material. The record can be fully viewed by clicking on the document title in the results list.
Unfortunately, indexing is not necessarily indicative of availability through the PAU library or through ILL. PAU availability is contingent on the content of the library's physical and electronic collections. ILL availability is contingent on the institutional policies and subscription terms and conditions to which other libraries must adhere when they subscribe to electronic content.
In some cases, research materials are indexed before they've actually been published. In other cases, there's a time lag between the publication date and the date an item is available for library access. This lag may be brief, to allow time for subscription-related processing. Longer lags are usually due to a publishing embargo. Some publishers impose embargoes on their electronic journals, meaning recent articles cannot be shared or even accessed by a subscribing library for a fixed time period. Libraries usually subscribe to electric journals with an embargo in order to provide access to very expensive titles at a lower price point. Embargoes can range in length from six months to five years following publication, but the typical duration is one year.
Additionally, some libraries won't deliver copies of digital content at all due to local policies and/or restrictive subscription terms. Libraries at major universities are often able to negotiate more favorable licensing terms for sharing electronic content, so they're in a better position to provide access when other libraries can. Although they usually charge a fee, the PAU Library covers the cost as part of its ILL services.
Research materials can be inaccessible for a number of different reasons.