You go to your doctor to get a check-up. A nurse wraps your arm with a pressure
cuff and pumps it up to measure your blood pressure which is recorded as two
numbers maybe 114/74. So far, so good. But what if you want to use that blood
pressure reading in a statistical analysis?
You can't enter 114/74 because it will be treated like a ratio of 150%
We need a single number that is usable for
statistics. For this example, we're going to use both Excel and SPSS as we dive
deeper into how to solve the tricky problem of isolating the data that we
need computing a new variable with a formula and then creating a single
number called the mean arterial pressure.
For this example we're going to start with some data in Excel. Now these data
come from patient charts and they represent blood pressure readings for a
collection of patients. We're going to isolate the parts of the data that we
need, and then use a formula to assemble those parts into a meaningful variable
that we can use to measure blood pressure. We are going to accomplish this
in three steps. Step 1: separate the values in Excel using the LEFT and RIGHT
commands. Step 2: bring that data into SPSS.
Step 3: calculate a new variable for MAP using the formula. Here is why we are
starting this process in Excel: I don't know of any way to do this in SPSS.
In Excel, however, we can use the LEFT and RIGHT functions to isolate each part of
the blood pressure reading. You, of course, recognize these as blood pressure
readings: systolic over diastolic. But Excel sees them as ratios, like 1.52%
We want the systolic numbers in one column and the diastolic
numbers in a second column, so that we can then use them in the mean arterial
pressure formula. =((2*DIA)+SYS)/3 And where did
I get this formula? The formula for mean arterial pressure is in most nursing
textbooks or you could find it with a quick internet search.
We will begin by creating some new columns labeled
SYS for systolic and DIA for diastolic. For the systolic numbers, we will use the
LEFT function. The syntax for the left function is this you first reference the
cell that contains the text, followed by the number of characters. The systolic
numbers are the first three numbers on the left so we would type
=LEFT (A2, 3). This creates a cell with the
first three numbers on the left end. Highlight the cell and double-click on
the handle to copy it to the remaining cells. If all of the diastolic numbers
are two digits, then the next part is easy. We simply use the RIGHT function to
copy only the rightmost two digits into the DIA column. This creates a cell with
the two numbers on the right end. Highlight the cell and double click on
the handle to copy it to the remaining cells. In our case however, we have
several blood pressure readings with three diastolic digits. Some have two
digits, some have three digits. What can we do? The solution is to improve our
diastolic formula. For the diastolic numbers we will still use the RIGHT
formula, referencing cell A2, but for the number of values we have Excel return
the entire length of A - - everything before the slash using the SEARCH
function.
Double click to copy down. LEFT and RIGHT functions belong in the category of TEXT
functions in Excel, meaning that the result of a LEFT or RIGHT formula is
always a text string, even if the original values were numbers; therefore,
these numbers are being read as strings. So if you try to do the mean arterial
pressure calculations in Excel, you will get an error. That is why at this point I
import these values into SPSS. After import, SPSS will read the numbers as
numeric values and import them that way. But for those of you who are wanting to
do this entirely in Excel, you should wrap the LEFT or RIGHT functions in the
VALUE function, which is designed to convert a string into a number.
I am going to save this Excel file to the desktop, so that I can import it into SPSS.
In SPSS I go to File -> Import Data -> Excel. I go to the Desktop, select the
blood pressure file and click Open.
I click OK in the import dialog box and the file opens. Notice that both systolic
and diastolic numbers have imported as scale data. To create the mean arterial
pressure, I will use the Compute Variable function with a formula. Go to Transform ->
-> Compute Variable. I will create a variable called MAP, and for the
variable label, I will type "Mean Arterial Pressure". The formula for MAP is
((2*DIA)+SYS)/3. Click OK. We now have a
single number calculated from blood pressure readings that we can use for
analyzing blood pressure among our respondents.