I am in the process of merging two data sets together in Stata and came up with a potential concern.
I am planning on sorting each data set in exactly the same manner on several categorical variables that are common to both sets of data. HOWEVER, several of the categorical variables have more categories present in one data set over the other. I have been careful enough to ensure that the coding matches up in both data sets (e.g. Red is coded as 1 in both data set A and B, but data set A has only Red, Green and Blue whereas data set B has Red, Green, Blue, and Yellow).
If I were to sort each data set the same way and generate an id variable (gen id = _n) and merge on that, would I run into any problems?
There is no statistical question here, as this is purely about data management in Stata, so I too shall shortly vote for this to be migrated to Stack Overflow, where I would be one of those who might try to answer it, so I will do that now.
What you describe to generate identifiers is not how to think of merging data sets, regardless of any of the other details in your question.
Imagine any two data sets, and then in each data set, generate an identifier that is based on the observation numbers, as you propose. Generating such similar identifiers does not create a genuine merge key. You might as well say that four values "Alan" "Bill" "Christopher" "David" in one data set can be merged with "William" "Xavier" "Yulia" "Zach" in another data set because both can be labelled with observation numbers 1 to 4.
My advice is threefold:
Try what you are proposing with your data and try to understand the results.
Consider whether you have something else altogether, namely an append problem. It is quite common to confuse the two.
If both of those fail, come back with a real problem and real code and real results for a small sample, rather than abstract worries.
I think I may have solved my problem - I figured I would post an answer specifically relating to the problem in case anybody has the same issue.
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I have two data sets: One containing information about the amount of time IT help spent at a customer and another data set with how much product a customer purchased. Both data sets contain unique ID numbers for each company and the fiscal quarter and year that link the sets together (e.g. ID# 1001 corresponds to the same company in both data sets). Additionally, the IT data set contains unique ID numbers for each IT person and the customer purchases data set contains a unique ID number for each purchase made. I am not interested in analysis at the individual employee level, so I collapsed the IT time data set to the total sum of time spent at a given company regardless of who was there.
I was interested in merging both data sets so that I could perform analysis to estimate some sort of "responsiveness" (or elasticity) function linking together IT time spent and products purchased.
I am certain this is a case of "merging" data because I want to add more VARIABLES not OBSERVATIONS - that is, I wish to horizontally elongate not vertically elongate my final data set.
Stata 12 has many options for merging - one to one, many to one, and one to many. Supposing that I treat my IT time data set as my master and my purchases data set as my merging set, I would perform a "m:1" or many to one merge. This is because I have MANY purchases corresponding to one observation per quarter per company.
Related
I am working on a project compiling data from a longitdinal study with approx 300 participants. The data I have been supplied with is divided into multiple SPSS-files, which all are in "long-format". Each data set contain the longitudinal information for one test. The participants have been measured at 20 time-points, so there is a maximum of 20 observations per participant.
I am to compile all these data sets into one SPSS-file in wide-format.
However, there are multiple challenges when combining these data sets into one data set:
Missing visits are not represented by a row in any of the dataset.
The response rate (i.e. total number of rows) differ between measure.
There are multiple errors in coding of visits (i.e. the "timpeoint" variable is wrong in many of the cases), but i have date of each visit.
To correct for two of these challanges have manually checked in one the databases, and corrected errorous "timepoint"-values, added missing rows. This "mother-database" is now of sound quality.
I was wondering if there is a way to merge the rest of the spss-files with this one, while meeting the following criteria:
Match by ID number.
Match by visit-date, using "mother-database" as a definition of a visit (note that not all visits are available in the data sets i am trying to merge with the "mother-database").
In case of missing data on a visit, add missing values.
In case of missing visit (i.e. no date), add missing values.
I have a site where customers purchase items that are tagged with a variety of taxonomy terms. I want to create a group of customers who might be interested in the same items by considering the tags associated with purchases they've made. Rather than comparing a list of tags for each customer each time I want to build the group, I'm wondering if I can use some type of scoring to solve the problem.
The way I'm thinking about it, each tag would have some unique number assigned to it. When I perform a scoring operation it would render a number that could only be achieved by combining a specific set of tags.
I could update a customer's "score" periodically so that it remains relevant.
Am I on the right track? Any ideas?
Your description of the problem looks much more like a clustering or recommendation problem. I am not sure if those tags are enough of an information to use clustering or recommendation tough.
Your idea of the score doesn't look promising to me, because the same sum could be achieved in several ways, if those numbers aren't carefully enough chosen.
What I would suggest you:
You can store tags for each user. When some user purchases a new item, you will add the tags of the item to the user's tags. On periodical time you will update the users profiles. Let's say we have users A and B. If at the time of the update the similarity between A and B is greater than some threshold, you will add a relation between the users which will indicate that the two users are similar. If it's lower you will remove the relation (if previously they were related). The similarity could be either a number of common tags or num_common_tags / num_of_tags_assigned_either_in_A_or_B.
Later on, when you will want to get users with particular set of tags, you will just do a query which checks which users have that set of tags. Also you can check for similar users to given user, just by looking up which users are linked with the user in question.
If you assign a unique power of two to each tag, then you can sum the values corresponding to the tags, and users with the exact same sets of tags will get identical values.
red = 1
green = 2
blue = 4
yellow = 8
For example, only customers who have the set of { red, blue } will have a value of 5.
This is essentially using a bitmap to represent a set. The drawback is that if you have many tags, you'll quickly run out of integers. For example, if your (unsigned) integer type is four bytes, you'd be limited to 32 tags. There are libraries and classes that let you represent much larger bitsets, but, at that point, it's probably worth considering other approaches.
Another problem with this approach is that it doesn't help you cluster members that are similar but not identical.
I'm working on a problem that involves reconciling data that represents estimates of the same system under two different classification hierarchies. I want to enforce the requirement that equivalent classes or groups of classes have the same sum.
For example, say Classification A divides industries into: Agriculture (sheep/cattle), Agriculture (non-sheep/cattle), Mining, Manufacturing (textiles), Manufacturing (non-textiles), ...
Meanwhile, Classification B has a different breakdown: Agriculture, Mining (iron ore), Mining (non-iron-ore), Manufacturing (chemical), Manufacturing (non-chemical), ...
In this case, any total for A_Agric_SheepCattle + A_Agric_NonSheepCattle should match the equivalent total for B_Agric; A_Mining should match B_MiningIronOre + B_Mining_NonIronOre; and A_MFG_Textiles+A_MFG_NonTextiles should match B_MFG_Chemical+B_MFG_NonChemical.
For bonus complication, one category may be involved in multiple equivalencies, e.g. B_Mining_IronOre might be involved in an equivalency with both A_Mining and A_Mining_Metallic.
I will be working with multi-dimensional tables, with this sort of concordance applied to more than one dimension - e.g. I might be compiling data on Industry x Product, so each equivalency will be used in multiple constraints; hence I need an efficient way to define them once and invoke repeatedly, instead of just setting a direct constraint "A_Agric_SheepCattle + A_Agric_NonSheepCattle = B_Agric".
The most natural way to represent this sort of concordance would seem to be as a list of pairs of sets. The catch is that the set sizes will vary - sometimes we have a 1:1 equivalence, sometimes it's "these 5 categories equate to those 7 categories", etc.
I found this related question which offers two answers for dealing with variable-sized sets. One is to define all set members in a single ordered set with indices, then define the starting index for each set within that. However, this seems unwieldy for my problem; both classifications are likely to be long, so I'd need to be hopping between two loooong lists of industries and two looong lists of indices to see a single equivalency. This seems like it would be a nuisance to check, and hard to modify (since any change to membership for one of the early sets changes the index numbers for all following sets).
The other is to define pairs of long fixed-length sets, and then pad each set to the required length with null members.
This would be a much better option for my purposes since it lets me eyeball a single line and see the equivalence that it represents. But it would require a LOT of padding; most of the equivalence groups will be small but a few might be quite large, and everything has to be padded to the size of the largest expected length.
Is there a better approach?
I'm trying to figure out an algorithm...
Input is a bunch of objects that have multiple values (eg 3 values per object, colour/taste/age, though it could be more).
The algorithm would then distribute the objects into a pre-defined number of sets. Each set should end up with almost the same number of objects (preferably the object count per set shouldn't differ more than 1), and achieve the objective of as fair a distribution of values per set as possible (eg try to have close to as many red in each set, and same for other colours, as well as tastes and ages, etc).
Values are tied to objects and cannot be changed. If you move an object from one set to another it brings all its values.
I found this related question: Algorithm for fair distribution of numbers into two sets
and the "number partitioning problem" suggested seems to help with single value distributions, but I'm looking for information/algorithms with multiple values per object (as described above).
Also note that the values cannot be normalized, ie each object cannot be totalled up into a single value.
Thank you kindly for any assistance.
IMHO, you should approach this as a clustering problem http://en.wikipedia.org/wiki/Cluster_analysis .
Currently I am loooking for a way to develop an algorithm which is supposed to analyse a large dataset (about 600M records). The records have parameters "calling party", "called party", "call duration" and I would like to create a graph of weighted connections among phone users.
The whole dataset consists of similar records - people mostly talk to their friends and don't dial random numbers but occasionaly a person calls "random" numbers as well. For analysing the records I was thinking about the following logic:
create an array of numbers to indicate the which records (row number) have already been scanned.
start scanning from the first line and for the first line combination "calling party", "called party" check for the same combinations in the database
sum the call durations and divide the result by the sum of all call durations
add the numbers of summed lines into the array created at the beginning
check the array if the next record number has already been summed
if it has already been summed then skip the record, else perform step 2
I would appreciate if anyone of you suggested any improvement of the logic described above.
p.s. the edges are directed therefore the (calling party, called party) is not equal to (called party, calling party)
Although the fact is not programming related I would like to emphasize that due to law and respect for user privacy all the informations that could possibly reveal the user identity have been hashed before the analysis.
As always with large datasets the more information you have about the distribution of values in them the better you can tailor an algorithm. For example, if you knew that there were only, say, 1000 different telephone numbers to consider you could create a 1000x1000 array into which to write your statistics.
Your first step should be to analyse the distribution(s) of data in your dataset.
In the absence of any further information about your data I'm inclined to suggest that you create a hash table. Read each record in your 600M dataset and calculate a hash address from the concatenation of calling and called numbers. Into the table at that address write the calling and called numbers (you'll need them later, and bear in mind that the hash is probably irreversible), add 1 to the number of calls and add the duration to the total duration. Repeat 600M times.
Now you have a hash table which contains the data you want.
Since there are 600 M records, it seems to be large enough to leverage a database (and not too large to require a distributed Database). So, you could simply load this into a DB (MySQL, SQLServer, Oracle, etc) and run the following queries:
select calling_party, called_party, sum(call_duration), avg(call_duration), min(call_duration), max (call_duration), count(*) from call_log group by calling_party, called_party order by 7 desc
That would be a start.
Next, you would want to run some Association analysis (possibly using Weka), or perhaps you would want to analyze this information as cubes (possibly using Mondrian/OLAP). If you tell us more, we can help you more.
Algorithmically, what the DB is doing internally is similar to what you would do yourself programmatically:
Scan each record
Find the record for each (calling_party, called_party) combination, and update its stats.
A good way to store and find records for (calling_party, called_party) would be to use a hashfunction and to find the matching record from the bucket.
Althought it may be tempting to create a two dimensional array for (calling_party, called_party), that will he a very sparse array (very wasteful).
How often will you need to perform this analysis? If this is a large, unique dataset and thus only once or twice - don't worry too much about the performance, just get it done, e.g. as Amrinder Arora says by using simple, existing tooling you happen to know.
You really want more information about the distribution as High Performance Mark says. For starters, it's be nice to know the count of unique phone numbers, the count of unique phone number pairs, and, the mean, variance and maximum of the count of calling/called phone numbers per unique phone number.
You really want more information about the analysis you want to perform on the result. For instance, are you more interested in holistic statistics or identifying individual clusters? Do you care more about following the links forward (determining who X frequently called) or following the links backward (determining who X was frequently called by)? Do you want to project overviews of this graph into low-dimensional spaces, i.e. 2d? Should be easy to indentify indirect links - e.g. X is near {A, B, C} all of whom are near Y so X is sorta near Y?
If you want fast and frequently adapted results, then be aware that a dense representation with good memory & temporal locality can easily make a huge difference in performance. In particular, that can easily outweigh a factor ln N in big-O notation; you may benefit from a dense, sorted representation over a hashtable. And databases? Those are really slow. Don't touch those if you can avoid it at all; they are likely to be a factor 10000 slower - or more, the more complex the queries are you want to perform on the result.
Just sort records by "calling party" and then by "called party". That way each unique pair will have all its occurrences in consecutive positions. Hence, you can calculate the weight of each pair (calling party, called party) in one pass with little extra memory.
For sorting, you can sort small chunks separately, and then do a N-way merge sort. That's memory efficient and can be easily parallelized.